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
dag_file = os.path.join(
            os.path.dirname(os.path.realpath(__file__)), '../dags/test_only_dummy_tasks.py'
        )
        
        dagbag = DagBag(dag_folder=dag_file, include_examples=False, read_dags_from_db=False)
        dagbag.sync_to_db()
        scheduler_job = SchedulerJob(subdir=os.devnull)
        scheduler_job.processor_agent = mock.MagicMock()
        dag = scheduler_job.dagbag.get_dag("test_only_dummy_tasks")
        
        session = settings.Session()
        orm_dag = session.query(DagModel).get(dag.dag_id)
        scheduler_job._create_dag_runs([orm_dag], session)
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1
        dr = drs[0]
        
        scheduler_job._schedule_dag_run(dr, {}, session)
        with create_session() as session:
            tis = session.query(TaskInstance).all()
        dags = scheduler_job.dagbag.dags.values()
        assert ['test_only_dummy_tasks'] == [dag.dag_id for dag in dags]
        assert 5 == len(tis)
        assert {
            ('test_task_a', 'success'),
            ('test_task_b', None),
            ('test_task_c', 'success'),
            ('test_task_on_execute', 'scheduled'),
            ('test_task_on_success', 'scheduled'),