workflows/tests: Start/Stop/Run Redis instead of MySQL
workflows/tests: Start/Stop/Run Redis instead of MySQL

- For reasons that I couldn't determine, no amount of sleeping was
  leaving enough time for MySQL to start - and debugging a GitHub
  Actions hosted runner is difficult. This problem also occurred on
  `master`, so it's not specifically related to my code changes here (PR
  288).
- As a sanity check for `brew services` working on GitHub Actions at
  all, I wondered if Redis would work - picked as random other `brew
  service` startable software - and it did!

Homebrew/homebrew-services