### 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
self.scheduler_job = SchedulerJob(subdir=os.devnull)
        self.scheduler_job.executor = MockExecutor(do_update=False)
        self.scheduler_job.processor_agent = mock.MagicMock(spec=DagFileProcessorAgent)
        num_queued = self.scheduler_job._do_scheduling(session)
        
        
        dag_run = session.merge(dag_run)
        session.refresh(dag_run)
        assert num_queued == 2
        assert dag_run.state == State.RUNNING
        
        dag_maker.create_dagrun(
            run_type=DagRunType.MANUAL,
            execution_date=DEFAULT_DATE + timedelta(hours=1),
            state=State.QUEUED,
            session=session,
        )
        session.flush()
        self.scheduler_job._do_scheduling(session)
        
        assert len(DagRun.find(dag_id=dag.dag_id, state=State.RUNNING, session=session)) == 1
        
        assert len(DagRun.find(dag_id=dag.dag_id, state=State.QUEUED, session=session)) == 1
    def test_max_active_runs_in_a_dag_doesnt_stop_running_dagruns_in_otherdags(self, dag_maker):
        session = settings.Session()
        with dag_maker(
            'test_dag1',
            start_date=DEFAULT_DATE,
            schedule_interval=timedelta(hours=1),
            max_active_runs=1,