### 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
date = dr.execution_date + timedelta(hours=1)
        
        date = timezone.datetime(2021, 1, 1)
        with dag_maker('test_dagrun_states_are_correct_3', start_date=date) as dag:
            DummyOperator(task_id='dummy_task')
        for i in range(16):
            dr = dag_maker.create_dagrun(run_id=f'dr3_run_{i+1}', state=State.RUNNING, execution_date=date)
            date = dr.execution_date + timedelta(hours=1)
        dr16 = DagRun.find(run_id='dr3_run_16')
        date = dr16[0].execution_date + timedelta(hours=1)
        for i in range(16, 32):
            dr = dag_maker.create_dagrun(run_id=f'dr2_run_{i+1}', state=State.QUEUED, execution_date=date)
            date = dr.execution_date + timedelta(hours=1)
        self.scheduler_job = SchedulerJob(subdir=os.devnull)
        self.scheduler_job.executor = MockExecutor(do_update=False)
        self.scheduler_job.processor_agent = mock.MagicMock(spec=DagFileProcessorAgent)
        ti = TaskInstance(task=task1, execution_date=DEFAULT_DATE)
        ti.refresh_from_db()
        ti.state = State.SUCCESS
        session.merge(ti)
        session.flush()
        
        with mock.patch.object(settings, "USE_JOB_SCHEDULE", False):
            self.scheduler_job._do_scheduling(session)
            self.scheduler_job._do_scheduling(session)
        assert DagRun.find(run_id='dr1_run_1')[0].state == State.SUCCESS
        assert DagRun.find(run_id='dr1_run_2')[0].state == State.RUNNING
    @pytest.mark.parametrize(
        "state, start_date, end_date",
        [