### 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 that if a dag run exists, scheduler._create_dag_runs does not raise an error.
        And if a Dag Run does not exist it creates next Dag Run. In both cases the Scheduler
        sets next execution date as DagModel.next_dagrun
        """
        with dag_maker(
            dag_id='test_scheduler_create_dag_runs_check_existing_run',
            schedule_interval=timedelta(days=1),
        ) as dag:
            DummyOperator(
                task_id='dummy',
            )
        session = settings.Session()
        assert dag.get_last_dagrun(session) is None
        dag_model = dag_maker.dag_model
        
        assert dag_model.next_dagrun == DEFAULT_DATE
        dagrun = dag_maker.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=dag_model.next_dagrun,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
            external_trigger=False,
            session=session,
            creating_job_id=2,
        )
        session.flush()
        assert dag.get_last_dagrun(session) == dagrun
        self.scheduler_job = SchedulerJob(subdir=os.devnull, executor=self.null_exec)