### 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 task instances in a pool that isn't full keep getting scheduled even when a pool is full.
        """
        with dag_maker(
            dag_id='test_scheduler_keeps_scheduling_pool_full_d1',
            start_date=DEFAULT_DATE,
        ):
            BashOperator(
                task_id='test_scheduler_keeps_scheduling_pool_full_t1',
                pool='test_scheduler_keeps_scheduling_pool_full_p1',
                bash_command='echo hi',
            )
        dag_d1 = dag_maker.dag
        with dag_maker(
            dag_id='test_scheduler_keeps_scheduling_pool_full_d2',
            start_date=DEFAULT_DATE,
        ):
            BashOperator(
                task_id='test_scheduler_keeps_scheduling_pool_full_t2',
                pool='test_scheduler_keeps_scheduling_pool_full_p2',
                bash_command='echo hi',
            )
        dag_d2 = dag_maker.dag
        session = settings.Session()
        pool_p1 = Pool(pool='test_scheduler_keeps_scheduling_pool_full_p1', slots=1)
        pool_p2 = Pool(pool='test_scheduler_keeps_scheduling_pool_full_p2', slots=10)
        session.add(pool_p1)
        session.add(pool_p2)
        session.flush()
        scheduler = SchedulerJob(executor=self.null_exec)