### 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
dag = self._get_dummy_dag(dag_maker, dag_id='test_backfill_conf')
        dag_maker.create_dagrun()
        executor = MockExecutor()
        conf_ = json.loads("""{"key": "value"}""")
        job = BackfillJob(
            dag=dag,
            executor=executor,
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=2),
            conf=conf_,
        )
        job.run()
        
        dr = DagRun.find(
            dag_id='test_backfill_conf', execution_start_date=DEFAULT_DATE + datetime.timedelta(days=1)
        )
        assert conf_ == dr[0].conf
    @patch('airflow.jobs.backfill_job.BackfillJob.log')
    def test_backfill_respect_max_active_tis_per_dag_limit(self, mock_log, dag_maker):