The test fails quite regularly - usually in one of many jobs:

Example


https://github.com/apache/airflow/pull/12850/checks?check_run_id=1506517544

```
______________ TestSchedulerJob.test_scheduler_task_start_date ________________

self = <tests.jobs.test_scheduler_job.TestSchedulerJob testMethod=test_scheduler_task_start_date>

    def test_scheduler_task_start_date(self):
        """
        Test that the scheduler respects task start dates that are different from DAG start dates
        """
    
        dagbag = DagBag(dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"), include_examples=False)
        dag_id = 'test_task_start_date_scheduling'
        dag = self.dagbag.get_dag(dag_id)
        dag.is_paused_upon_creation = False
        dagbag.bag_dag(dag=dag, root_dag=dag)
    
        # Deactivate other dags in this file so the scheduler doesn't waste time processing them
        other_dag = self.dagbag.get_dag('test_start_date_scheduling')
        other_dag.is_paused_upon_creation = True
        dagbag.bag_dag(dag=other_dag, root_dag=other_dag)
    
        dagbag.sync_to_db()
    
        scheduler = SchedulerJob(executor=self.null_exec, subdir=dag.fileloc, num_runs=2)
        scheduler.run()
    
        session = settings.Session()
        tiq = session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id)
        ti1s = tiq.filter(TaskInstance.task_id == 'dummy1').all()
        ti2s = tiq.filter(TaskInstance.task_id == 'dummy2').all()
        self.assertEqual(len(ti1s), 0)
>       self.assertEqual(len(ti2s), 2)
E       AssertionError: 1 != 2

tests/jobs/test_scheduler_job.py:2415: AssertionError


```

tests/jobs/test_scheduler_job.py
dag_file_processor = DagFileProcessor(dag_ids=[], log=mock.MagicMock())
        dag_file_processor.manage_slas(dag=dag, session=session)
        sla_callback.assert_not_called()
    def test_dag_file_processor_sla_miss_callback_exception(self):
        """
        Test that the dag file processor gracefully logs an exception if there is a problem
        calling the sla_miss_callback
        """
        session = settings.Session()
        sla_callback = MagicMock(side_effect=RuntimeError('Could not call function'))
        test_start_date = days_ago(2)
        dag = DAG(
            dag_id='test_sla_miss',
            sla_miss_callback=sla_callback,
            default_args={'start_date': test_start_date},
        )
        task = DummyOperator(task_id='dummy', dag=dag, owner='airflow', sla=datetime.timedelta(hours=1))
        session.merge(TaskInstance(task=task, execution_date=test_start_date, state='Success'))
        
        session.merge(SlaMiss(task_id='dummy', dag_id='test_sla_miss', execution_date=test_start_date))
        
        mock_log = mock.MagicMock()
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock_log)
        dag_file_processor.manage_slas(dag=dag, session=session)
        assert sla_callback.called
        mock_log.exception.assert_called_once_with(
            'Could not call sla_miss_callback for DAG %s', 'test_sla_miss'
        )
    @mock.patch('airflow.jobs.scheduler_job.send_email')
    def test_dag_file_processor_only_collect_emails_from_sla_missed_tasks(self, mock_send_email):