MySQL tests are being skipped in CI

[rkm]

Just spotted this in a recent build log. All tests against MySQL are being silently skipped.

DatabaseTests.GetCleanedServer (from RDMP) is marking tests as Inconclusive (without a message) if the database doesn't exist, so we can't see the underlying reason. It's likely to be an invalid connection string though, since MySQL is definitely running in the CI.

We need some way of indicating to the base DatabaseTests class that we don't want to silently skip some tests, like we have in RequiresExternalService in this repo.