### 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
assert len(drs) == 1
        dr = drs[0]
        
        self.scheduler_job._schedule_dag_run(dr, session)
        with create_session() as session:
            tis = session.query(TaskInstance).all()
        dags = self.scheduler_job.dagbag.dags.values()
        assert ['test_only_dummy_tasks'] == [dag.dag_id for dag in dags]
        assert 5 == len(tis)
        assert {
            ('test_task_a', 'success'),
            ('test_task_b', None),
            ('test_task_c', 'success'),
            ('test_task_on_execute', 'scheduled'),
            ('test_task_on_success', 'scheduled'),
        } == {(ti.task_id, ti.state) for ti in tis}
        for state, start_date, end_date, duration in [
            (ti.state, ti.start_date, ti.end_date, ti.duration) for ti in tis
        ]:
            if state == 'success':
                assert start_date is not None
                assert end_date is not None
                assert 0.0 == duration
            else:
                assert start_date is None
                assert end_date is None
                assert duration is None
        self.scheduler_job._schedule_dag_run(dr, session)
        with create_session() as session:
            tis = session.query(TaskInstance).all()