### 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
"""This tests that queued dagruns stops creating once max_active_runs is reached"""
        with dag_maker(max_active_runs=10) as dag:
            DummyOperator(task_id='mytask')
        session = settings.Session()
        self.scheduler_job = SchedulerJob(subdir=os.devnull)
        self.scheduler_job.executor = MockExecutor()
        self.scheduler_job.processor_agent = mock.MagicMock()
        self.scheduler_job.dagbag = dag_maker.dagbag
        session = settings.Session()
        orm_dag = session.query(DagModel).get(dag.dag_id)
        assert orm_dag is not None
        for _ in range(20):
            self.scheduler_job._create_dag_runs([orm_dag], session)
        drs = session.query(DagRun).all()
        assert len(drs) == 10
        for dr in drs:
            dr.state = State.RUNNING
            session.merge(dr)
        session.commit()
        assert session.query(DagRun.state).filter(DagRun.state == State.RUNNING).count() == 10
        for _ in range(20):
            self.scheduler_job._create_dag_runs([orm_dag], session)
        assert session.query(DagRun).count() == 10
        assert session.query(DagRun.state).filter(DagRun.state == State.RUNNING).count() == 10
        assert session.query(DagRun.state).filter(DagRun.state == State.QUEUED).count() == 0
        assert orm_dag.next_dagrun_create_after is None
    def test_dagrun_timeout_verify_max_active_runs(self, dag_maker):