Quarantine issuable_list_spec.rb

What does this MR do and why?

This MR quarantines spec/features/issuables/issuable_list_spec.rb:51 in order to improve the stability of the master pipeline. This particular test has been identified as flaky in the below master-broken incidents:

• gitlab-org/quality/engineering-productivity/master-broken-incidents#1001 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#1007 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#1014 (closed)

Related to #393344

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.