Quarantine non-deterministic test

Also, add an extra expectation for when the test is de-quarantined.

qa/qa/specs/features/ee/browser_ui/2_plan/multiple_assignees_for_issues/two_assignees_spec.rb

 # frozen_string_literal: true
 
 module QA
-  context 'Plan' do
+  # https://gitlab.com/gitlab-org/quality/nightly/issues/146
+  context 'Plan', :quarantine do
     describe 'Multiple assignees per issue' do
       before do
         Runtime::Browser.visit(:gitlab, Page::Main::Login)