The `TestSchedulerJob.test_retry_still_in_executor` fails occasionally and should be quarantined.

```
  ________________ TestSchedulerJob.test_retry_still_in_executor _________________
  
  self = <tests.jobs.test_scheduler_job.TestSchedulerJob object at 0x7f4c9031f128>
  
      def test_retry_still_in_executor(self):
          """
          Checks if the scheduler does not put a task in limbo, when a task is retried
          but is still present in the executor.
          """
          executor = MockExecutor(do_update=False)
          dagbag = DagBag(dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"), include_examples=False)
          dagbag.dags.clear()
      
          dag = DAG(dag_id='test_retry_still_in_executor', start_date=DEFAULT_DATE, schedule_interval="@once")
          dag_task1 = BashOperator(
              task_id='test_retry_handling_op', bash_command='exit 1', retries=1, dag=dag, owner='airflow'
          )
      
          dag.clear()
          dag.is_subdag = False
      
          with create_session() as session:
              orm_dag = DagModel(dag_id=dag.dag_id)
              orm_dag.is_paused = False
              session.merge(orm_dag)
      
          dagbag.bag_dag(dag=dag, root_dag=dag)
          dagbag.sync_to_db()
      
          @mock.patch('airflow.dag_processing.processor.DagBag', return_value=dagbag)
          def do_schedule(mock_dagbag):
              # Use a empty file since the above mock will return the
              # expected DAGs. Also specify only a single file so that it doesn't
              # try to schedule the above DAG repeatedly.
              self.scheduler_job = SchedulerJob(
                  num_runs=1, executor=executor, subdir=os.path.join(settings.DAGS_FOLDER, "no_dags.py")
              )
              self.scheduler_job.heartrate = 0
              self.scheduler_job.run()
      
          do_schedule()
          with create_session() as session:
              ti = (
                  session.query(TaskInstance)
                  .filter(
                      TaskInstance.dag_id == 'test_retry_still_in_executor',
                      TaskInstance.task_id == 'test_retry_handling_op',
                  )
                  .first()
              )
  >       ti.task = dag_task1
  E       AttributeError: 'NoneType' object has no attribute 'task'
  
  tests/jobs/test_scheduler_job.py:2514: AttributeError
```
tests/test_utils/asserts.py
event.remove(airflow.settings.engine, "after_cursor_execute", self.after_cursor_execute)
        log.debug("Queries count: %d", sum(self.result.values()))
    def after_cursor_execute(self, *args, **kwargs):
        stack = [
            f
            for f in traceback.extract_stack()
            if 'sqlalchemy' not in f.filename
            and __file__ != f.filename
            and ('session.py' not in f.filename and f.name != 'wrapper')
        ]
        stack_info = ">".join([f"{f.filename.rpartition('/')[-1]}:{f.name}:{f.lineno}" for f in stack][-3:])
        self.result[f"{stack_info}"] += 1
count_queries = CountQueries
@contextmanager
def assert_queries_count(expected_count, message_fmt=None):
    with count_queries() as result:
        yield None
    count = sum(result.values())
    if expected_count != count:
        message_fmt = (
            message_fmt
            or "The expected number of db queries is {expected_count}. "
            "The current number is {current_count}.\n\n"
            "Recorded query locations:"
        )
        message = message_fmt.format(current_count=count, expected_count=expected_count)
        for location, count in result.items():
            message += f'\n\t{location}:\t{count}'
        raise AssertionError(message)