Stop node before removing the notification file to make sure the command has been terminated. After then we could removing those files safely and do not receive any permission error. (See #14446)

The permission error is Windows specific, documented in python doc:
>On Windows, attempting to remove a file that is in use causes an exception to be raised

See https://docs.python.org/3/library/os.html#os.remove

ninja
test_runner.py feature_notifications