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/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"