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
"""
        Check that new DAG files are picked up, and import errors recorded.
        This is more of an "integration" test as it checks SchedulerJob, DagFileProcessorManager and
        DagFileProcessor
        """
        dags_folder = mkdtemp()
        try:
            unparseable_filename = os.path.join(dags_folder, TEMP_DAG_FILENAME)
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
            with open(unparseable_filename, 'w') as unparseable_file:
                unparseable_file.writelines(UNPARSEABLE_DAG_FILE_CONTENTS)
            print("Second run")
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
        finally:
            shutil.rmtree(dags_folder)
        with create_session() as session:
            import_errors = session.query(errors.ImportError).all()
        assert len(import_errors) == 1
        import_error = import_errors[0]
        assert import_error.filename == unparseable_filename
        assert import_error.stacktrace == f"invalid syntax ({TEMP_DAG_FILENAME}, line 1)"
    def test_no_import_errors_with_parseable_dag(self):