0 tests should be considered a failure

[polishmatt]

If no tests are run that should be considered a failure and return a non-zero exit code. The most likely scenarios for this are a misconfigured environment (where the user should be notified that something needs to be fixed) and a manual test discovery with no results (where the exit code doesn't matter).

There appear to be no valid scenarios for running 0 tests to be considered a success but there should still be an option to restore the previous behavior.