message: Fix test breakage on Mac Python3 due to assert_called()
  Replace assert_called() with self.assertTrue and called().
diff: @@ -237,7 +237,7 @@ class _TrainingExecutorTrainingTest(object):
  config=test.mock.ANY,
  start=False)
  - mock_server_instance.start.assert_called()
  + self.assertTrue(mock_server_instance.start.called)
  mock_est.train.assert_called_with(input_fn=train_spec.input_fn,
  max_steps=train_spec.max_steps,
  @@ -349,7 +349,7 @@ class TrainingExecutorRunWorkerTest(_TrainingExecutorTrainingTest,
  with test.mock.patch.object(time, 'sleep') as mock_sleep:
  mock_sleep.side_effect = lambda s: self.assertEqual(expected_secs, s)
  self._run_task(executor)
  - mock_sleep.assert_called()
  + self.assertTrue(mock_sleep.called)
  class TrainingExecutorRunChiefTest(_TrainingExecutorTrainingTest,
  