Update test to put back a removed method

Also, add a comment explaining the need of it, and a TODO to make
the test completely independent.

qa/qa/specs/features/browser_ui/2_plan/ee_epic/promote_issue_to_epic_spec.rb

@@ -23,6 +23,10 @@ module QA
         end
 
         Page::Project::Issue::Show.perform do |show|
+          # Due to the randomness of tests execution, sometimes a previous test
+          # may have changed the filter, which makes the below action needed.
+          # TODO: Make this test completely independent, not requiring the below step.
+          show.select_all_activities_filter
           show.comment('/promote')
 
           expect(show).to have_content("promoted to epic")