Improve test reliability by resetting the pre-state in test_redisqueue

[sturmianseq]

This PR aims to improve the reliability of the test test_mock_queue_connection by cleaning the pre-state.
The test can fail if the pre-state is polluted:

>       assert mock_queue.connected is False
E       assert True is False
E        +  where True = <redisqueue.mock.MockRedisQueue object at 0x7f42458d4130>.connected

tests/test_redisqueue.py:46: AssertionError