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/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']")