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 that the scheduler can successfully queue multiple dags in parallel
        """
        dag_ids = ['test_start_date_scheduling', 'test_dagrun_states_success']
        for dag_id in dag_ids:
            dag = self.dagbag.get_dag(dag_id)
            dag.clear()
        scheduler = SchedulerJob(
            executor=self.null_exec,
            subdir=os.path.join(TEST_DAG_FOLDER, 'test_scheduler_dags.py'),
            num_runs=1,
        )
        scheduler.run()
        
        dag_id = 'test_start_date_scheduling'
        session = settings.Session()
        assert len(session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id).all()) == 0
    @conf_vars({("core", "mp_start_method"): "spawn"})
    def test_scheduler_multiprocessing_with_spawn_method(self):