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
clear_db_sla_miss,
    set_default_pool_slots,
)
from tests.test_utils.mock_executor import MockExecutor
from tests.test_utils.mock_operators import CustomOperator
ROOT_FOLDER = os.path.realpath(
    os.path.join(os.path.dirname(os.path.realpath(__file__)), os.pardir, os.pardir)
)
PERF_DAGS_FOLDER = os.path.join(ROOT_FOLDER, "tests", "test_utils", "perf", "dags")
ELASTIC_DAG_FILE = os.path.join(PERF_DAGS_FOLDER, "elastic_dag.py")
TEST_DAG_FOLDER = os.environ['AIRFLOW__CORE__DAGS_FOLDER']
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
TRY_NUMBER = 1
@pytest.fixture(scope="class")
def disable_load_example():
    with conf_vars({('core', 'load_examples'): 'false'}):
        with env_vars({('core', 'load_examples'): 'false'}):
            yield
@pytest.fixture(scope="module")
def dagbag():
    from airflow.models.dagbag import DagBag
    
    non_serialized_dagbag = DagBag(read_dags_from_db=False, include_examples=False)
    non_serialized_dagbag.sync_to_db()
    return DagBag(read_dags_from_db=True)
@pytest.mark.usefixtures("disable_load_example")
@pytest.mark.need_serialized_dag
class TestSchedulerJob:
    @staticmethod
    def clean_db():