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/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