Quarantine user_sees_discussions_navigation_spec.rb

What does this MR do and why?

This MR quarantines spec/features/merge_request/user_sees_discussions_navigation_spec.rb to help improve the stability of the master pipeline. This test has been reported as flaky in the below master-broken incidents:

• gitlab-org/quality/engineering-productivity/master-broken-incidents#918 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#995 (closed)

Related to #391912 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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