### 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
end_date=DEFAULT_DATE + datetime.timedelta(days=7),
        )
        job.run()
        assert len(executor.history) > 0
        concurrency_limit_reached_at_least_once = False
        num_running_task_instances = 0
        for running_task_instances in executor.history:
            assert len(running_task_instances) <= dag.max_active_tasks
            num_running_task_instances += len(running_task_instances)
            if len(running_task_instances) == dag.max_active_tasks:
                concurrency_limit_reached_at_least_once = True
        assert 8 == num_running_task_instances
        assert 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_task_concurrency_limit_reached_in_debug
        assert times_dag_concurrency_limit_reached_in_debug > 0
    @patch('airflow.jobs.backfill_job.BackfillJob.log')
    def test_backfill_respect_default_pool_limit(self, mock_log, dag_maker):