Quarantine user_comments_on_diff_spec due to flakiness

spec/features/merge_request/user_comments_on_diff_spec.rb

@@ -136,7 +136,7 @@ RSpec.describe 'User comments on a diff', :js do
         add_comment('-13', '+15')
       end
 
-      it 'allows comments on previously hidden lines at the top of a file' do
+      it 'allows comments on previously hidden lines at the top of a file', quarantine: 'https://gitlab.com/gitlab-org/gitlab/-/issues/285294' do
         # Click -9, expand up, select 1 add and verify comment
         page.within('[data-path="files/ruby/popen.rb"]') do
           all('.js-unfold-all')[0].click