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
assert scheduled_tis == 2
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1
        dr = drs[0]
        tis = dr.get_task_instances()
        assert len(tis) == 2
    def test_runs_respected_after_clear(self):
        """
        Test if _process_task_instances only schedules ti's up to max_active_runs
        (related to issue AIRFLOW-137)
        """
        dag = DAG(dag_id='test_scheduler_max_active_runs_respected_after_clear', start_date=DEFAULT_DATE)
        dag.max_active_runs = 3
        BashOperator(task_id='dummy', dag=dag, owner='airflow', bash_command='echo Hi')
        session = settings.Session()
        orm_dag = DagModel(dag_id=dag.dag_id)
        session.merge(orm_dag)
        session.commit()
        session.close()
        dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        scheduler = SchedulerJob(subdir=os.devnull)
        scheduler.processor_agent = mock.MagicMock()
        scheduler.dagbag.bag_dag(dag, root_dag=dag)
        dag.clear()
        date = DEFAULT_DATE
        dr1 = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=date,
            state=State.RUNNING,
        )