Quarantine tree_show_spec.rb

What does this MR do and why?

This MR quarantines spec/features/projects/tree/tree_show_spec.rb:162 to help to improve the stability of the master pipeline. This test was identified as a flaky spec in the following master-broken incidents:

• gitlab-org/quality/engineering-productivity/master-broken-incidents#908 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#909 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#915 (closed)

Related to #391780

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.