Don't run tests when wrong xdebug mode is set

[staabm]

doing a process restart and persisting all possible cli args etc, while not just re-using composer/xdebug-handler and re-implementing everything is not a useful way and requires lots of code we would need to maintain.

therefore I figured I would just make sure that phpunit exits early instead of running all tests, when the coverage driver cannot be activated

closes https://github.com/sebastianbergmann/phpunit/issues/6233

[staabm]

should be good to go

[sebastianbergmann]

Merged manually, thanks.

[staabm]

Thanks 🙏