Quarantine user_sees_merge_request_file_tree_sidebar_spec

What does this MR do and why?

This MR quarantines user_sees_merge_request_file_tree_sidebar_spec.rb from recent master broken incident: gitlab-org/quality/engineering-productivity/master-broken-incidents#4270 (closed).

Related to #432665 (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.