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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 -->


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,
            deferrable=True,
        )
    @mock.patch("airflow.providers.google.cloud.operators.dataflow.DataflowHook")
    def test_execute(self, mock_dataflow, sync_operator):
        sync_operator.execute(mock.MagicMock())
        mock_dataflow.assert_called_once_with(
            gcp_conn_id="google_cloud_default",
            drain_pipeline=False,
            cancel_timeout=600,
            wait_until_finished=None,
            impersonation_chain=None,
        )
        mock_dataflow.return_value.start_flex_template.assert_called_once_with(
            body={"launchParameter": TEST_FLEX_PARAMETERS},
            location=TEST_LOCATION,
            project_id=TEST_PROJECT,
            on_new_job_callback=mock.ANY,
        )
    def test_on_kill(self, sync_operator):