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
"""
        Test task instances in a pool that isn't full keep getting scheduled even when a pool is full.
        """
        with dag_maker(
            dag_id='test_scheduler_keeps_scheduling_pool_full_d1',
            start_date=DEFAULT_DATE,
        ):
            BashOperator(
                task_id='test_scheduler_keeps_scheduling_pool_full_t1',
                pool='test_scheduler_keeps_scheduling_pool_full_p1',
                bash_command='echo hi',
            )
        dag_d1 = dag_maker.dag
        with dag_maker(
            dag_id='test_scheduler_keeps_scheduling_pool_full_d2',
            start_date=DEFAULT_DATE,
        ):
            BashOperator(
                task_id='test_scheduler_keeps_scheduling_pool_full_t2',
                pool='test_scheduler_keeps_scheduling_pool_full_p2',
                bash_command='echo hi',
            )
        dag_d2 = dag_maker.dag
        session = settings.Session()
        pool_p1 = Pool(pool='test_scheduler_keeps_scheduling_pool_full_p1', slots=1)
        pool_p2 = Pool(pool='test_scheduler_keeps_scheduling_pool_full_p2', slots=10)
        session.add(pool_p1)
        session.add(pool_p2)
        session.flush()
        scheduler = SchedulerJob(executor=self.null_exec)