### 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
if runs:
            run = runs[0]
            if run.state == State.RUNNING:
                respect_dag_max_active_limit = False
        else:
            run = None
        
        
        if respect_dag_max_active_limit and current_active_dag_count >= dag.max_active_runs:
            return None
        run = run or dag.create_dagrun(
            execution_date=run_date,
            data_interval=dagrun_info.data_interval,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
            external_trigger=False,
            session=session,
            conf=self.conf,
            run_type=DagRunType.BACKFILL_JOB,
            creating_job_id=self.id,
        )
        
        run.dag = dag
        
        run.state = State.RUNNING
        run.run_type = DagRunType.BACKFILL_JOB
        run.verify_integrity(session=session)
        return run
    @provide_session
    def _task_instances_for_dag_run(self, dag_run, session=None):