### 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.
airflow/jobs/backfill_job.py
"""
        Returns a dag run for the given run date, which will be matched to an existing
        dag run if available or create a new dag run otherwise. If the max_active_runs
        limit is reached, this function will return None.
        :param dagrun_info: Schedule information for the dag run
        :param dag: DAG
        :param session: the database session object
        :return: a DagRun in state RUNNING or None
        """
        run_date = dagrun_info.logical_date
        
        respect_dag_max_active_limit = bool(dag.timetable.can_run and not dag.is_subdag)
        current_active_dag_count = dag.get_num_active_runs(external_trigger=False)
        
        
        runs = DagRun.find(dag_id=dag.dag_id, execution_date=run_date, session=session)
        run: Optional[DagRun]