### 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
default_pool_slots = 2
        set_default_pool_slots(default_pool_slots)
        dag = self._get_dummy_dag(dag_maker, dag_id='test_backfill_with_no_pool_limit')
        dag_maker.create_dagrun()
        executor = MockExecutor()
        job = BackfillJob(
            dag=dag,
            executor=executor,
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=7),
        )
        job.run()
        assert len(executor.history) > 0
        default_pool_task_slot_count_reached_at_least_once = False
        num_running_task_instances = 0
        
        
        
        for running_task_instances in executor.history:
            assert len(running_task_instances) <= default_pool_slots
            num_running_task_instances += len(running_task_instances)
            if len(running_task_instances) == default_pool_slots:
                default_pool_task_slot_count_reached_at_least_once = True
        assert 8 == num_running_task_instances
        assert default_pool_task_slot_count_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(