Unittest `test_cancel` deadlocks for PBS-based schedulers

[teojgo]

The unittest deadlocks since the test might by cancelled before the job starts executing. In that case, no stdout or stderr is produced and therefore done == False:
https://github.com/eth-cscs/reframe/blob/625ce43d724c13343c6c693066a0db3ee4fb3c5e/reframe/core/schedulers/torque.py#L97