Test: quarantine flaky user_sees_merge_request_file_tree_sidebar_spec.rb

What does this MR do and why?

Causing master-broken incidents

• gitlab-org/quality/engineering-productivity/master-broken-incidents#4285 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#4272 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#4271 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#4270 (closed)

How to set up and validate locally

rspec ./spec/features/merge_request/user_sees_merge_request_file_tree_sidebar_spec.rb

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.