Fix CI race condition

[Arnaud Morin]

We recently had two issues in CI:

mistral.tests.unit.engine.test_subworkflows_pause_resume.SubworkflowPauseResumeTest.test_pause_resume_cascade_down_to_with_items_subworkflowstesttools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/mistral/mistral/tests/unit/engine/test_subworkflows_pause_resume.py", line 975, in test_pause_resume_cascade_down_to_with_items_subworkflows
    self.assertEqual(states.RUNNING, wf_2_ex_1_task_1_ex.state)
  File "/home/zuul/src/opendev.org/openstack/mistral/.tox/unit-postgresql/lib/python3.10/site-packages/testtools/testcase.py", line 395, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/home/zuul/src/opendev.org/openstack/mistral/.tox/unit-postgresql/lib/python3.10/site-packages/testtools/testcase.py", line 482, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: 'RUNNING' != 'IDLE'

And:

mistral.tests.unit.engine.test_policies.PoliciesTest.test_timeout_policy_success_after_timeouttesttools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/mistral/mistral/tests/unit/engine/test_policies.py", line 1505, in test_timeout_policy_success_after_timeout
    self.assertEqual(states.IDLE, task_ex.state)
  File "/home/zuul/src/opendev.org/openstack/mistral/.tox/unit-mysql/lib/python3.10/site-packages/testtools/testcase.py", line 395, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/home/zuul/src/opendev.org/openstack/mistral/.tox/unit-mysql/lib/python3.10/site-packages/testtools/testcase.py", line 482, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: 'IDLE' != 'RUNNING'

We should try to fix that

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/mistral/+/906437

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/mistral/+/906595

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Arnaud Morin <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/mistral/+/906437
Reason: dup https://review.opendev.org/c/openstack/mistral/+/906595