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/models/image_spec.rb

@@ -5,7 +5,7 @@ describe Image do
   it_behaves_like "image validations", "proposal_image"
 
   it "stores attachments with Active Storage" do
-    image = create(:image, attachment: File.new("spec/fixtures/files/clippy.jpg"))
+    image = create(:image, attachment: File.new(file_fixture("clippy.jpg")))
 
     expect(image.attachment).to be_attached
     expect(image.attachment.filename).to eq "clippy.jpg"