### 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(executor=self.null_exec, subdir=dag.fileloc, num_runs=1)
            self.scheduler_job.run()
            
            assert len(session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id).all()) == 0
            session.commit()
            assert [] == self.null_exec.sorted_tasks
            
            
            
            
            bf_exec = MockExecutor()
            backfill = BackfillJob(executor=bf_exec, dag=dag, start_date=DEFAULT_DATE, end_date=DEFAULT_DATE)
            backfill.run()
            
            assert len(session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id).all()) == 1
            assert [
                (
                    TaskInstanceKey(dag.dag_id, 'dummy', f'backfill__{DEFAULT_DATE.isoformat()}', 1),
                    (State.SUCCESS, None),
                ),
            ] == bf_exec.sorted_tasks
            session.commit()
            self.scheduler_job = SchedulerJob(dag.fileloc, executor=self.null_exec, num_runs=1)
            self.scheduler_job.run()
            
            assert len(session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id).all()) == 1
            session.commit()
            assert [] == self.null_exec.sorted_tasks
    @pytest.mark.quarantined
    def test_scheduler_task_start_date(self):