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/test_utils/asserts.py
#
#
import logging
import re
import traceback
from collections import Counter
from contextlib import contextmanager
from sqlalchemy import event
import airflow.settings
log = logging.getLogger(__name__)
def assert_equal_ignore_multiple_spaces(case, first, second, msg=None):
    def _trim(s):
        return re.sub(r"\s+", " ", s.strip())
    return case.assertEqual(_trim(first), _trim(second), msg)
class CountQueries:
    """
    Counts the number of queries sent to Airflow Database in a given context.
    Does not support multiple processes. When a new process is started in context, its queries will
    not be included.
    """
    def __init__(self):
        self.result = Counter()
    def __enter__(self):
        event.listen(airflow.settings.engine, "after_cursor_execute", self.after_cursor_execute)
        return self.result
    def __exit__(self, type_, value, tb):