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/models/test_taskinstance.py
email='test@test.test',
            on_success_callback=callback_wrapper.success_handler,
            dag=dag,
        )
        ti = TI(task=task, execution_date=datetime.datetime.now())
        ti.state = State.RUNNING
        session = settings.Session()
        session.merge(ti)
        dag.create_dagrun(
            execution_date=ti.execution_date,
            state=State.RUNNING,
            run_type=DagRunType.SCHEDULED,
            session=session,
        )
        session.commit()
        callback_wrapper.wrap_task_instance(ti)
        ti._run_raw_task()
        ti._run_finished_callback()
        assert callback_wrapper.callback_ran
        assert callback_wrapper.task_state_in_callback == State.SUCCESS
        ti.refresh_from_db()
        assert ti.state == State.SUCCESS
    @staticmethod
    def _test_previous_dates_setup(
        schedule_interval: Union[str, datetime.timedelta, None], catchup: bool, scenario: List[str]
    ) -> list:
        dag_id = 'test_previous_dates'
        dag = models.DAG(dag_id=dag_id, schedule_interval=schedule_interval, catchup=catchup)
        task = DummyOperator(task_id='task', dag=dag, start_date=DEFAULT_DATE)
        def get_test_ti(session, execution_date: pendulum.DateTime, state: str) -> TI: