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
on_failure_callback=mock_on_failure_2,
            on_retry_callback=mock_on_retry_2,
            retries=1,
            dag=dag,
        )
        ti2 = TI(task=task2, execution_date=start_date)
        ti2.state = State.FAILED
        ti2.handle_failure("test retry handling")
        ti2._run_finished_callback()
        mock_on_failure_2.assert_not_called()
        context_arg_2 = mock_on_retry_2.call_args[0][0]
        assert context_arg_2 and "task_instance" in context_arg_2
        
        mock_on_failure_3 = mock.MagicMock()
        mock_on_retry_3 = mock.MagicMock()
        task3 = DummyOperator(
            task_id="test_handle_failure_on_force_fail",
            on_failure_callback=mock_on_failure_3,
            on_retry_callback=mock_on_retry_3,
            retries=1,
            dag=dag,
        )
        ti3 = TI(task=task3, execution_date=start_date)
        ti3.state = State.FAILED
        ti3.handle_failure("test force_fail handling", force_fail=True)
        ti3._run_finished_callback()
        context_arg_3 = mock_on_failure_3.call_args[0][0]
        assert context_arg_3 and "task_instance" in context_arg_3
        mock_on_retry_3.assert_not_called()
    def test_does_not_retry_on_airflow_fail_exception(self):