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
for file_path in list_py_file_paths(TEST_DAG_FOLDER, include_examples=False):
            detected_files.add(file_path)
        assert detected_files == expected_files
        ignored_files = {
            'helper.py',
        }
        example_dag_folder = airflow.example_dags.__path__[0]
        for root, _, files in os.walk(example_dag_folder):  
            for file_name in files:
                if file_name.endswith('.py') or file_name.endswith('.zip'):
                    if file_name not in ['__init__.py'] and file_name not in ignored_files:
                        expected_files.add(os.path.join(root, file_name))
        detected_files.clear()
        for file_path in list_py_file_paths(TEST_DAG_FOLDER, include_examples=True):
            detected_files.add(file_path)
        assert detected_files == expected_files
        smart_sensor_dag_folder = airflow.smart_sensor_dags.__path__[0]
        for root, _, files in os.walk(smart_sensor_dag_folder):
            for file_name in files:
                if (file_name.endswith('.py') or file_name.endswith('.zip')) and file_name not in [
                    '__init__.py'
                ]:
                    expected_files.add(os.path.join(root, file_name))
        detected_files.clear()
        for file_path in list_py_file_paths(
            TEST_DAG_FOLDER, include_examples=True, include_smart_sensor=True
        ):
            detected_files.add(file_path)
        assert detected_files == expected_files
    def test_adopt_or_reset_orphaned_tasks_nothing(self):