Quarantine user_comments_on_diff_spec.rb

What does this MR do and why?

This MR quarantines spec/features/merge_request/user_comments_on_diff_spec.rb to help improve the stability of the master pipeline.

This test has been reported as flaky in the recent master-broken incident: gitlab-org/quality/engineering-productivity/master-broken-incidents#1013 (closed)

Related to #393518

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.