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
dag2.sync_to_db(session=session)
        ti1 = TaskInstance(task1, DEFAULT_DATE)
        ti1.state = State.QUEUED
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.FAILED, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        assert ti1.state == State.QUEUED
        mock_task_callback.assert_called_once_with(
            full_filepath='/test_path1/',
            simple_task_instance=mock.ANY,
            msg='Executor reports task instance '
            '<TaskInstance: test_process_executor_events.dummy_task 2016-01-01 00:00:00+00:00 [queued]> '
            'finished (failed) although the task says its queued. (Info: None) '
            'Was the task killed externally?',
        )
        scheduler.processor_agent.send_callback_to_execute.assert_called_once_with(task_callback)
        scheduler.processor_agent.reset_mock()
        
        ti1.state = State.SUCCESS
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.SUCCESS, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        assert ti1.state == State.SUCCESS
        scheduler.processor_agent.send_callback_to_execute.assert_not_called()
        mock_stats_incr.assert_called_once_with('scheduler.tasks.killed_externally')
    def test_process_executor_events_uses_inmemory_try_number(self):