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
"""
        Test if a a dagrun will not be scheduled if max_dag_runs
        has been reached and dagrun_timeout is not reached
        Test if a a dagrun would be scheduled if max_dag_runs has
        been reached but dagrun_timeout is also reached
        """
        dag = DAG(dag_id='test_scheduler_verify_max_active_runs_and_dagrun_timeout', start_date=DEFAULT_DATE)
        dag.max_active_runs = 1
        dag.dagrun_timeout = datetime.timedelta(seconds=60)
        DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        scheduler = SchedulerJob(subdir=os.devnull)
        scheduler.dagbag.bag_dag(dag, root_dag=dag)
        scheduler.dagbag.sync_to_db()
        session = settings.Session()
        orm_dag = session.query(DagModel).get(dag.dag_id)
        assert orm_dag is not None
        dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        scheduler = SchedulerJob(subdir=os.devnull)
        scheduler._create_dag_runs([orm_dag], session)
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1