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 scheduler._create_dag_runs does not raise an error when the DAG does not exist
        in serialized_dag table
        """
        dag = DAG(dag_id='test_scheduler_create_dag_runs_does_not_raise_error', start_date=DEFAULT_DATE)
        DummyOperator(
            task_id='dummy',
            dag=dag,
        )
        dagbag = DagBag(
            dag_folder=os.devnull,
            include_examples=False,
            read_dags_from_db=False,
        )
        dagbag.bag_dag(dag=dag, root_dag=dag)
        
        DAG.bulk_write_to_db(dagbag.dags.values())
        dag_model = DagModel.get_dagmodel(dag.dag_id)
        scheduler = SchedulerJob(subdir=os.devnull, executor=self.null_exec)
        scheduler.processor_agent = mock.MagicMock()
        with create_session() as session, self.assertLogs(
            'airflow.jobs.scheduler_job', level="ERROR"
        ) as log_output:
            scheduler._create_dag_runs([dag_model], session)
            assert (
                "airflow.exceptions.SerializedDagNotFound: DAG "
                "'test_scheduler_create_dag_runs_does_not_raise_error' not found in serialized_dag table"
            ) in log_output.output[0]
    def test_bulk_write_to_db_external_trigger_dont_skip_scheduled_run(self):