### 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._do_scheduling(session)
        dr1 = dag.get_dagrun(DEFAULT_DATE, session=session)
        assert dr1 is not None
        assert dr1.state == State.RUNNING
        assert dr1.execution_date == DEFAULT_DATE
        assert dr1.data_interval_start == DEFAULT_DATE
        assert dr1.data_interval_end == DEFAULT_DATE + timedelta(minutes=1)
        
        dag_model = session.query(DagModel).get(dag.dag_id)
        assert dag_model.next_dagrun == DEFAULT_DATE + timedelta(minutes=1)
        assert dag_model.next_dagrun_data_interval_start == DEFAULT_DATE + timedelta(minutes=1)
        assert dag_model.next_dagrun_data_interval_end == DEFAULT_DATE + timedelta(minutes=2)
        
        dr = dag.create_dagrun(
            state=State.RUNNING,
            execution_date=timezone.utcnow(),
            run_type=DagRunType.MANUAL,
            session=session,
            external_trigger=True,
        )
        assert dr is not None
        
        DAG.bulk_write_to_db([dag], session)
        
        
        dag_model = session.query(DagModel).get(dag.dag_id)
        assert dag_model.next_dagrun == DEFAULT_DATE + timedelta(minutes=1)
        assert dag_model.next_dagrun_data_interval_start == DEFAULT_DATE + timedelta(minutes=1)
        assert dag_model.next_dagrun_data_interval_end == DEFAULT_DATE + timedelta(minutes=2)
    def test_scheduler_create_dag_runs_check_existing_run(self, dag_maker):