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/lib/tasks/active_storage_spec.rb

@@ -35,7 +35,7 @@ describe "active storage tasks" do
     end
 
     it "does not modify old ckeditor attachments" do
-      image = Ckeditor::Picture.create!(data: Rack::Test::UploadedFile.new("spec/fixtures/files/clippy.png"))
+      image = Ckeditor::Picture.create!(data: Rack::Test::UploadedFile.new(file_fixture("clippy.png")))
 
       expect(image.storage_data.attachment.name).to eq "storage_data"