Fix flakey time-sensitive specs that rely on notes background Sidekiq jobs

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/48093

We use `perform_enqueued_jobs` because it is "adapter-agnostic so it's better to use it over Sidekiq::Testing.inline!." -- https://gitlab.com/gitlab-org/gitlab-ce/issues/22991

spec/support/helpers/features/notes_helpers.rb

@@ -13,7 +13,7 @@ module Spec
       module Features
         module NotesHelpers
           def add_note(text)
-            Sidekiq::Testing.fake! do
+            perform_enqueued_jobs do
               page.within(".js-main-target-form") do
                 fill_in("note[note]", with: text)
                 find(".js-comment-submit-button").click