test_expired_messages randomly failed

[Feilong Wang]

zaqar.tests.unit.storage.test_impl_mongodb.MongodbFIFOMessageTests.test_expired_messages failed randomly with below error:

[x]

ft14.7: zaqar.tests.unit.storage.test_impl_mongodb.MongodbFIFOMessageTests.test_expired_messages_StringException: Empty attachments:
  pythonlogging:'zaqar'
  stderr
  stdout

Traceback (most recent call last):
  File "/home/jenkins/workspace/gate-zaqar-python35/zaqar/tests/helpers.py", line 271, in wrapper
    test_method(self)
  File "/home/jenkins/workspace/gate-zaqar-python35/zaqar/tests/unit/storage/base.py", line 601, in test_expired_messages
    self.fail("Didn't remove the queue")
  File "/home/jenkins/workspace/gate-zaqar-python35/.tox/py35/lib/python3.5/site-packages/unittest2/case.py", line 690, in fail
    raise self.failureException(msg)
AssertionError: Didn't remove the queue