Describe the new feature:
This is a follow-up issue from #34550. It would be great if we could merge servo and servo driver WPT runners and run them in the WPT workflow. So tests that require testdriver.js can be tested instead of skipped. I'd like to help but I'm not too familiar with whole wptrunner setup yet.

And @jdm what does your comment about --debugger mean? Is it about opening GDB while running WPT?