This tests fails occasionally on mysql at least:

https://github.com/apache/airflow/pull/11555/checks?check_run_id=1260840314#step:9:1125

```
=================================== FAILURES ===================================
_____________ TestSchedulerJob.test_no_orphan_process_will_be_left _____________

self = <tests.jobs.test_scheduler_job.TestSchedulerJob testMethod=test_no_orphan_process_will_be_left>

    def test_no_orphan_process_will_be_left(self):
        empty_dir = mkdtemp()
        current_process = psutil.Process()
        old_children = current_process.children(recursive=True)
        scheduler = SchedulerJob(subdir=empty_dir,
                                 num_runs=1,
                                 executor=MockExecutor(do_update=False))
        scheduler.run()
        shutil.rmtree(empty_dir)
    
        # Remove potential noise created by previous tests.
        current_children = set(current_process.children(recursive=True)) - set(
            old_children)
>       self.assertFalse(current_children)
E       AssertionError: {psutil.Process(pid=3474, name='/usr/local/bin/', status='running', started='19:26:22')} is not false

tests/jobs/test_scheduler_job.py:871: AssertionError
```



tests/jobs/test_scheduler_job.py
clear_db_runs()
        clear_db_pools()
        clear_db_dags()
        clear_db_sla_miss()
        clear_db_import_errors()
        clear_db_jobs()
        clear_db_serialized_dags()
    def setUp(self):
        self.clean_db()
        
        
        self.null_exec = MockExecutor()
    def tearDown(self) -> None:
        self.clean_db()
    def create_test_dag(self, start_date=DEFAULT_DATE, end_date=DEFAULT_DATE + timedelta(hours=1), **kwargs):
        dag = DAG(
            dag_id='test_scheduler_reschedule',
            start_date=start_date,
            
            end_date=end_date,
        )
        dag.clear()
        dag.is_subdag = False
        with create_session() as session:
            orm_dag = DagModel(dag_id=dag.dag_id, is_paused=False)
            session.merge(orm_dag)
            session.commit()
        return dag
    @classmethod
    def setUpClass(cls):