Use the file_fixture helper in tests

This way we don't have to write `"spec/fixtures/files"` every time.

Note this method isn't included in factories. We could include it like
so:

```
FactoryBot::SyntaxRunner.class_eval do
  include ActiveSupport::Testing::FileFixtures
  self.file_fixture_path = RSpec.configuration.file_fixture_path
end
```

However, I'm not sure about the possible side effects, and since we only
use attachments in a few factories, there isn't much gain in applying
the monkey-patch.

spec/system/admin/budget_phases_spec.rb

@@ -46,7 +46,7 @@ describe "Admin budget phases" do
     scenario "shows successful notice when updating the phase with a valid image" do
       visit edit_admin_budget_budget_phase_path(budget, budget.current_phase)
 
-      imageable_attach_new_file(Rails.root.join("spec/fixtures/files/clippy.jpg"))
+      imageable_attach_new_file(file_fixture("clippy.jpg"))
 
       click_on "Save changes"
 

spec/system/admin/legislation/processes_spec.rb

@@ -175,7 +175,7 @@ describe "Admin collaborative legislation", :admin do
         fill_in "End", with: base_date + 5.days
       end
 
-      imageable_attach_new_file(Rails.root.join("spec/fixtures/files/clippy.jpg"))
+      imageable_attach_new_file(file_fixture("clippy.jpg"))
 
       click_button "Create process"
 

spec/system/admin/local_census_records/imports_spec.rb

@@ -1,7 +1,7 @@
 require "rails_helper"
 
 describe "Imports", :admin do
-  let(:base_files_path) { %w[spec fixtures files local_census_records import] }
+  let(:base_files_path) { "local_census_records/import/" }
 
   describe "New" do
     scenario "Should show import form" do
@@ -17,7 +17,7 @@ describe "Imports", :admin do
     scenario "Should show success notice after successful import" do
       within "form#new_local_census_records_import" do
         path = base_files_path << "valid.csv"
-        file = File.join(Rails.root, *path)
+        file = file_fixture(path)
         attach_file("local_census_records_import_file", file)
         click_button "Save"
       end
@@ -35,8 +35,7 @@ describe "Imports", :admin do
 
     scenario "Should show alert when file is not supported" do
       within "form#new_local_census_records_import" do
-        path = ["spec", "fixtures", "files", "clippy.jpg"]
-        file = File.join(Rails.root, *path)
+        file = file_fixture("clippy.jpg")
         attach_file("local_census_records_import_file", file)
         click_button "Save"
       end
@@ -47,7 +46,7 @@ describe "Imports", :admin do
     scenario "Should show successfully created local census records at created group" do
       within "form#new_local_census_records_import" do
         path = base_files_path << "valid.csv"
-        file = File.join(Rails.root, *path)
+        file = file_fixture(path)
         attach_file("local_census_records_import_file", file)
         click_button "Save"
       end
@@ -59,7 +58,7 @@ describe "Imports", :admin do
     scenario "Should show invalid local census records at errored group" do
       within "form#new_local_census_records_import" do
         path = base_files_path << "invalid.csv"
-        file = File.join(Rails.root, *path)
+        file = file_fixture(path)
         attach_file("local_census_records_import_file", file)
         click_button "Save"
       end
@@ -71,7 +70,7 @@ describe "Imports", :admin do
     scenario "Should show error messages inside cells at errored group" do
       within "form#new_local_census_records_import" do
         path = base_files_path << "invalid.csv"
-        file = File.join(Rails.root, *path)
+        file = file_fixture(path)
         attach_file("local_census_records_import_file", file)
         click_button "Save"
       end

spec/system/admin/poll/questions/answers/images/images_spec.rb

@@ -38,7 +38,7 @@ describe "Images", :admin do
     expect(page).not_to have_content("clippy.jpg")
 
     visit new_admin_answer_image_path(answer)
-    imageable_attach_new_file(Rails.root.join("spec/fixtures/files/clippy.jpg"))
+    imageable_attach_new_file(file_fixture("clippy.jpg"))
     click_button "Save image"
 
     expect(page).to have_css("img[title='clippy.jpg']")

spec/system/admin/site_customization/documents_spec.rb

@@ -54,7 +54,7 @@ describe "Documents", :admin do
   scenario "Create" do
     visit new_admin_site_customization_document_path
 
-    attach_file("document_attachment", "#{Rails.root}/spec/fixtures/files/logo.pdf")
+    attach_file("document_attachment", file_fixture("logo.pdf"))
     click_button "Upload"
 
     expect(page).to have_content "Document uploaded succesfully"

spec/system/admin/site_customization/images_spec.rb

@@ -10,7 +10,7 @@ describe "Admin custom images", :admin do
     end
 
     within("tr#image_logo_header") do
-      attach_file "site_customization_image_image", "spec/fixtures/files/logo_header.png"
+      attach_file "site_customization_image_image", file_fixture("logo_header.png")
       click_button "Update"
     end
 
@@ -22,7 +22,7 @@ describe "Admin custom images", :admin do
     visit admin_site_customization_images_path
 
     within("tr#image_map") do
-      attach_file "site_customization_image_image", "spec/fixtures/files/custom_map.jpg"
+      attach_file "site_customization_image_image", file_fixture("custom_map.jpg")
       click_button "Update"
     end
 
@@ -37,7 +37,7 @@ describe "Admin custom images", :admin do
     visit admin_site_customization_images_path
 
     within("tr#image_map") do
-      attach_file "site_customization_image_image", "spec/fixtures/files/custom_map.jpg"
+      attach_file "site_customization_image_image", file_fixture("custom_map.jpg")
       click_button "Update"
     end
 
@@ -63,7 +63,7 @@ describe "Admin custom images", :admin do
   scenario "Custom apple touch icon is replaced on front views" do
     create(:site_customization_image,
            name: "apple-touch-icon-200",
-           image: File.new("spec/fixtures/files/apple-touch-icon-custom-200.png"))
+           image: File.new(file_fixture("apple-touch-icon-custom-200.png")))
 
     visit root_path
 
@@ -77,7 +77,7 @@ describe "Admin custom images", :admin do
     visit admin_site_customization_images_path
 
     within("tr#image_logo_email") do
-      attach_file "site_customization_image_image", "spec/fixtures/files/logo_email_custom.png"
+      attach_file "site_customization_image_image", file_fixture("logo_email_custom.png")
       click_button "Update"
     end
 
@@ -92,7 +92,7 @@ describe "Admin custom images", :admin do
     visit admin_site_customization_images_path
 
     within("tr#image_social_media_icon") do
-      attach_file "site_customization_image_image", "spec/fixtures/files/logo_header.png"
+      attach_file "site_customization_image_image", file_fixture("logo_header.png")
       click_button "Update"
     end
 
@@ -104,7 +104,7 @@ describe "Admin custom images", :admin do
     visit admin_site_customization_images_path
 
     within("tr#image_social_media_icon") do
-      attach_file "site_customization_image_image", "spec/fixtures/files/social_media_icon.png"
+      attach_file "site_customization_image_image", file_fixture("social_media_icon.png")
       click_button "Update"
     end
 

spec/system/admin/widgets/cards_spec.rb

@@ -251,7 +251,7 @@ describe "Cards", :admin do
 
   def attach_image_to_card
     click_link "Add image"
-    attach_file "Choose image", Rails.root.join("spec/fixtures/files/clippy.jpg")
+    attach_file "Choose image", file_fixture("clippy.jpg")
 
     expect(page).to have_field("widget_card_image_attributes_title", with: "clippy.jpg")
   end

spec/system/ckeditor_spec.rb

@@ -33,7 +33,7 @@ describe "CKEditor" do
     click_link "Upload"
 
     within_frame(1) do
-      attach_file "Send it to the Server", Rails.root.join("spec/fixtures/files/clippy.jpg")
+      attach_file "Send it to the Server", file_fixture("clippy.jpg")
     end
 
     click_link "Send it to the Server"

spec/system/legislation/proposals_spec.rb

@@ -146,7 +146,7 @@ describe "Legislation Proposals" do
 
     fill_in "Proposal title", with: "Legislation proposal with image"
     fill_in "Proposal summary", with: "Including an image on a legislation proposal"
-    imageable_attach_new_file(Rails.root.join("spec/fixtures/files/clippy.jpg"))
+    imageable_attach_new_file(file_fixture("clippy.jpg"))
     check "legislation_proposal_terms_of_service"
     click_button "Create proposal"