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:

1. Visit a merge request page.
2. Verify that the query counts are present.
3. Verify that the gitaly counts are present.
4. Verify that AJAX requests are available to switch to and have the same counts.

And then add from there if we find future issues.