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/models/test_taskinstance.py
"""
        Test that get_previous_start_date() can handle TaskInstance with no start_date.
        """
        with DAG("test_get_previous_start_date_none", start_date=DEFAULT_DATE, schedule_interval=None) as dag:
            task = DummyOperator(task_id="op")
        day_1 = DEFAULT_DATE
        day_2 = DEFAULT_DATE + datetime.timedelta(days=1)
        
        
        
        dagrun_1 = dag.create_dagrun(
            execution_date=day_1,
            state=State.RUNNING,
            run_type=DagRunType.MANUAL,
        )
        dagrun_2 = dag.create_dagrun(
            execution_date=day_2,
            state=State.RUNNING,
            run_type=DagRunType.MANUAL,
        )
        ti_1 = dagrun_1.get_task_instance(task.task_id)
        ti_2 = dagrun_2.get_task_instance(task.task_id)
        ti_1.task = task
        ti_2.task = task
        assert ti_2.get_previous_start_date() == ti_1.start_date
        assert ti_1.start_date is None
    def test_pendulum_template_dates(self):