[lldb] Fix and re-enable TestMultipleDebuggers

The comment says that TestMultipleDebuggers was XFAILed because it was
failing nondeterministically in which case it should be skipped not
failed (as XPASS will cause the test suite to fail).

The reason it fails is because it was not marked as a no-debug-info test
case. I've ran the test in a loop and it has been passing consistently.
Let's enable it and see if the bots agree, if not we can skip it.