The test fails occasionally with the below stacktrace, so I am marking this as Quarantined.

```
  _______________ TestSchedulerJob.test_scheduler_verify_pool_full _______________
  
  self = <tests.jobs.test_scheduler_job.TestSchedulerJob object at 0x7fbaaaba0f40>
  
      def test_scheduler_verify_pool_full(self):
          """
          Test task instances not queued when pool is full
          """
          dag = DAG(dag_id='test_scheduler_verify_pool_full', start_date=DEFAULT_DATE)
      
          BashOperator(
              task_id='dummy',
              dag=dag,
              owner='airflow',
              pool='test_scheduler_verify_pool_full',
              bash_command='echo hi',
          )
      
          dagbag = DagBag(
              dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"),
              include_examples=False,
              read_dags_from_db=True,
          )
          dagbag.bag_dag(dag=dag, root_dag=dag)
          dagbag.sync_to_db()
      
          session = settings.Session()
          pool = Pool(pool='test_scheduler_verify_pool_full', slots=1)
          session.add(pool)
          session.flush()
      
          dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
          SerializedDagModel.write_dag(dag)
      
          self.scheduler_job = SchedulerJob(executor=self.null_exec)
          self.scheduler_job.processor_agent = mock.MagicMock()
      
          # Create 2 dagruns, which will create 2 task instances.
          dr = dag.create_dagrun(
              run_type=DagRunType.SCHEDULED,
              execution_date=DEFAULT_DATE,
              state=State.RUNNING,
          )
  >       self.scheduler_job._schedule_dag_run(dr, session)
  
  tests/jobs/test_scheduler_job.py:2108: 
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  airflow/jobs/scheduler_job.py:1020: in _schedule_dag_run
      dag = dag_run.dag = self.dagbag.get_dag(dag_run.dag_id, session=session)
  airflow/utils/session.py:67: in wrapper
      return func(*args, **kwargs)
  airflow/models/dagbag.py:186: in get_dag
      self._add_dag_from_db(dag_id=dag_id, session=session)
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  
  self = <airflow.models.dagbag.DagBag object at 0x7fbaaab95100>
  dag_id = 'test_scheduler_verify_pool_full'
  session = <sqlalchemy.orm.session.Session object at 0x7fbaaad9d0a0>
  
      def _add_dag_from_db(self, dag_id: str, session: Session):
          """Add DAG to DagBag from DB"""
          from airflow.models.serialized_dag import SerializedDagModel
      
          row = SerializedDagModel.get(dag_id, session)
          if not row:
  >           raise SerializedDagNotFound(f"DAG '{dag_id}' not found in serialized_dag table")
  E           airflow.exceptions.SerializedDagNotFound: DAG 'test_scheduler_verify_pool_full' not found in serialized_dag table
```
tests/jobs/test_scheduler_job.py
"""
        Test that Operator links are not loaded inside the Scheduling Loop (that does not include
        DagFileProcessorProcess) especially the critical loop of the Scheduler.
        This is to avoid running User code in the Scheduler and prevent any deadlocks
        """
        with dag_maker(dag_id='test_extra_operator_links_not_loaded_in_scheduler') as dag:
            
            _ = CustomOperator(task_id='custom_task')
        custom_task = dag.task_dict['custom_task']
        
        assert custom_task.operator_extra_links
        session = settings.Session()
        self.scheduler_job = SchedulerJob(executor=self.null_exec)
        self.scheduler_job.processor_agent = mock.MagicMock()
        self.scheduler_job._start_queued_dagruns(session)
        session.flush()
        
        s_dag_2 = self.scheduler_job.dagbag.get_dag(dag.dag_id)
        custom_task = s_dag_2.task_dict['custom_task']
        
        assert not custom_task.operator_extra_links
    def test_scheduler_create_dag_runs_does_not_raise_error(self, caplog, dag_maker):