Add QA tests for the performance bar
The performance bar needs QA tests because it's possible for a change in workhorse to break it (https://gitlab.com/gitlab-org/gitlab-ce/issues/60111), and people are unlikely to notice in day-to-day development.
We can add simple tests, like:
- Visit a merge request page.
- Verify that the query counts are present.
- Verify that the gitaly counts are present.
- Verify that AJAX requests are available to switch to and have the same counts.
And then add from there if we find future issues.