### Body

The `test_no_orphan_process_will_be_left` in `TestSchedulerJob` is really flaky.

Before we know how to fix it, I will quarantine it:

https://github.com/apache/airflow/pull/18691/checks?check_run_id=3811660138

```
  =================================== FAILURES ===================================
  _____________ TestSchedulerJob.test_no_orphan_process_will_be_left _____________
  
  self = <tests.jobs.test_scheduler_job.TestSchedulerJob object at 0x7fc2f91d2ac8>
  
      def test_no_orphan_process_will_be_left(self):
          empty_dir = mkdtemp()
          current_process = psutil.Process()
          old_children = current_process.children(recursive=True)
          self.scheduler_job = SchedulerJob(
              subdir=empty_dir, num_runs=1, executor=MockExecutor(do_update=False)
          )
          self.scheduler_job.run()
          shutil.rmtree(empty_dir)
      
          # Remove potential noise created by previous tests.
          current_children = set(current_process.children(recursive=True)) - set(old_children)
  >       assert not current_children
  E       AssertionError: assert not {psutil.Process(pid=2895, name='pytest', status='running', started='06:53:45')}
  
```

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
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():