The full error was

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ERROR: test_task_executor_execute (units.executor.test_task_executor.TestTaskExecutor)
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Traceback (most recent call last):
  File "/home/mg/src/ansible/test/units/executor/test_task_executor.py", line 252, in test_task_executor_execute
    mock_action.run.return_value = dict(ansible_facts=dict())
  File "/home/mg/src/ansible/lib/ansible/executor/task_executor.py", line 317, in _execute
    if self._task.async > 0:
TypeError: unorderable types: MagicMock() > int()

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Experiments show that Python 2 MagicMock() > 0 is true, so I'm setting
the async property on mock_task to 1.  (If I set it to 0, the test fails
anyway.)