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**Description**

Related to the discussion here: https://github.com/apache/airflow/pull/8553#discussion_r416136954

Allow user to define expected terminal state. e.g. `JOB_STATE_CANCELLED` could be not treated as failure but expected terminal state.


<!-- A short description of your feature -->


init_kwargs = {
            "task_id": TASK_ID,
            "template": TEMPLATE,
            "location": TEST_LOCATION,
            "poll_sleep": POLL_SLEEP,
            "wait_until_finished": True,
            "cancel_timeout": CANCEL_TIMEOUT,
        }
        operator = DataflowTemplatedJobStartOperator(**init_kwargs)
        operator.execute(None)
        assert dataflow_mock.called
        _, kwargs = dataflow_mock.call_args_list[0]
        assert not kwargs["variables"]
        assert kwargs["location"] == TEST_LOCATION
class TestDataflowStartFlexTemplateOperator:
    @pytest.fixture
    def sync_operator(self):
        return DataflowStartFlexTemplateOperator(
            task_id="start_flex_template_streaming_beam_sql",
            body={"launchParameter": TEST_FLEX_PARAMETERS},
            do_xcom_push=True,
            project_id=TEST_PROJECT,
            location=TEST_LOCATION,
        )
    @pytest.fixture
    def deferrable_operator(self):