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/budget_phases_spec.rb

@@ -46,7 +46,7 @@ describe "Admin budget phases" do
     scenario "shows successful notice when updating the phase with a valid image" do
       visit edit_admin_budget_budget_phase_path(budget, budget.current_phase)
 
-      imageable_attach_new_file(Rails.root.join("spec/fixtures/files/clippy.jpg"))
+      imageable_attach_new_file(file_fixture("clippy.jpg"))
 
       click_on "Save changes"