message: Fix assert_called error on Python3
  by replacing it with assertTrue(....called)
diff: @@ -626,7 +626,7 @@ class _TrainingExecutorTrainingTest(object):
  self._run_task(training._TrainingExecutor(mock_est, mock_train_spec,
  mock_eval_spec))
  - mock_est.train.assert_called()
  + self.assertTrue(mock_est.train.called)
  mock_server.assert_not_called()
  def test_fail_with_empty_task_type(self):
  @@ -836,7 +836,7 @@ class TrainingExecutorRunMasterTest(test.TestCase):
  executor.run_master()
  mock_server.assert_not_called()
  - mock_est.train.assert_called()
  + self.assertTrue(mock_est.train.called)
  def test_fail_with_empty_task_type(self):
  mock_est = test.mock.Mock(spec=estimator_lib.Estimator)
  