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
date = dag.following_schedule(date)
        dr2 = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=date,
            state=State.RUNNING,
        )
        date = dag.following_schedule(date)
        dr3 = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=date,
            state=State.RUNNING,
        )
        
        assert dr1 is not None
        assert dr2 is not None
        assert dr3 is not None
        assert len(DagRun.find(dag_id=dag.dag_id, state=State.RUNNING, session=session)) == 3
        
        dag.max_active_runs = 1
        
        
        with create_session() as session:
            num_scheduled = scheduler._schedule_dag_run(dr1, set(), session)
            assert num_scheduled == 1
            num_scheduled = scheduler._schedule_dag_run(dr2, {dr1.execution_date}, session)
            assert num_scheduled == 0
            num_scheduled = scheduler._schedule_dag_run(dr3, {dr1.execution_date}, session)
            assert num_scheduled == 0
    @patch.object(TaskInstance, 'handle_failure_with_callback')
    def test_execute_on_failure_callbacks(self, mock_ti_handle_failure):