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/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"