This runs ember-test-audit for UI PRs to compare how long
the tests take for the base vs the PR. It posts or updates
a comparison of times and test count.

It’s somewhat slow to report back as it runs the test suite
three times to even out variability in a shared environment.
If we end up being unhappy with that slowness, we could try
running the repetitions in parallel as well, but that would
involve more changes to ember-test-audit.