### 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
"""
        Test various invariants of _create_dag_runs.
        - That the run created has the creating_job_id set
        - That the run created is on QUEUED State
        - That dag_model has next_dagrun
        """
        with dag_maker(dag_id='test_create_dag_runs') as dag:
            DummyOperator(task_id='dummy')
        dag_model = dag_maker.dag_model
        self.scheduler_job = SchedulerJob(executor=self.null_exec)
        self.scheduler_job.processor_agent = mock.MagicMock()
        with create_session() as session:
            self.scheduler_job._create_dag_runs([dag_model], session)
        dr = session.query(DagRun).filter(DagRun.dag_id == dag.dag_id).first()
        
        assert dr.state == State.QUEUED
        assert dr.start_date is None
        assert dag.get_last_dagrun().creating_job_id == self.scheduler_job.id
    @freeze_time(DEFAULT_DATE + datetime.timedelta(days=1, seconds=9))
    @mock.patch('airflow.jobs.scheduler_job.Stats.timing')
    def test_start_dagruns(self, stats_timing, dag_maker):