### 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
with dag_maker(
            dag_id='test_backfill_ordered_concurrent_execute',
            schedule_interval="@daily",
        ) as dag:
            op1 = DummyOperator(task_id='leave1')
            op2 = DummyOperator(task_id='leave2')
            op3 = DummyOperator(task_id='upstream_level_1')
            op4 = DummyOperator(task_id='upstream_level_2')
            op5 = DummyOperator(task_id='upstream_level_3')
            
            op2.set_downstream(op3)
            op1.set_downstream(op3)