[infra] Rename test.py option to --cleanup-dart-processes
[infra] Rename test.py option to --cleanup-dart-processes

The option is changed from --cleanup_dart_processes to
--cleanup-dart-processes.

Change-Id: I3f9aefea81ac4ea040741d0850a851688f69e1a2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170084
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk