### Body

Add unit test for changes in https://github.com/apache/airflow/pull/17305

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
tests/jobs/test_backfill_job.py
num_running_task_instances += len(running_task_instances)
            if len(running_task_instances) == max_active_tis_per_dag:
                task_concurrency_limit_reached_at_least_once = True
        assert 8 == num_running_task_instances
        assert task_concurrency_limit_reached_at_least_once
        times_dag_concurrency_limit_reached_in_debug = self._times_called_with(
            mock_log.debug,
            DagConcurrencyLimitReached,
        )
        times_pool_limit_reached_in_debug = self._times_called_with(
            mock_log.debug,
            NoAvailablePoolSlot,
        )
        times_task_concurrency_limit_reached_in_debug = self._times_called_with(
            mock_log.debug,
            TaskConcurrencyLimitReached,
        )
        assert 0 == times_pool_limit_reached_in_debug
        assert 0 == times_dag_concurrency_limit_reached_in_debug
        assert times_task_concurrency_limit_reached_in_debug > 0
    @patch('airflow.jobs.backfill_job.BackfillJob.log')
    def test_backfill_respect_dag_concurrency_limit(self, mock_log, dag_maker):
        dag = self._get_dummy_dag(dag_maker, dag_id='test_backfill_respect_concurrency_limit')
        dag_maker.create_dagrun()
        dag.max_active_tasks = 2
        executor = MockExecutor()
        job = BackfillJob(
            dag=dag,
            executor=executor,
            start_date=DEFAULT_DATE,