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/jobs/test_scheduler_job.py
BashOperator(task_id='bash_task_1', dag=dag, bash_command='echo hi')
        SerializedDagModel.write_dag(dag=dag)
        dag_version_2 = SerializedDagModel.get_latest_version_hash(dr.dag_id, session=session)
        assert dag_version_2 != dag_version_1
        self.scheduler_job._schedule_dag_run(dr, session)
        session.flush()
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1
        dr = drs[0]
        assert dr.dag_hash == dag_version_2
        assert self.scheduler_job.dagbag.dags == {'test_verify_integrity_if_dag_changed': dag}
        assert len(self.scheduler_job.dagbag.dags.get("test_verify_integrity_if_dag_changed").tasks) == 2
        tis_count = (
            session.query(func.count(TaskInstance.task_id))
            .filter(
                TaskInstance.dag_id == dr.dag_id,
                TaskInstance.execution_date == dr.execution_date,
                TaskInstance.state == State.SCHEDULED,
            )
            .scalar()
        )
        assert tis_count == 2
        latest_dag_version = SerializedDagModel.get_latest_version_hash(dr.dag_id, session=session)
        assert dr.dag_hash == latest_dag_version
        session.rollback()
        session.close()
    @pytest.mark.quarantined
    @pytest.mark.need_serialized_dag
    def test_retry_still_in_executor(self, dag_maker):