### 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 to check that a DAG returns its active runs
        """
        now = timezone.utcnow()
        six_hours_ago_to_the_hour = (now - datetime.timedelta(hours=6)).replace(
            minute=0, second=0, microsecond=0
        )
        start_date = six_hours_ago_to_the_hour
        dag_name1 = 'get_active_runs_test'
        default_args = {'depends_on_past': False, 'start_date': start_date}
        with dag_maker(
            dag_name1, schedule_interval='* * * * *', max_active_runs=1, default_args=default_args
        ) as dag1:
            run_this_1 = DummyOperator(task_id='run_this_1')
            run_this_2 = DummyOperator(task_id='run_this_2')
            run_this_2.set_upstream(run_this_1)
            run_this_3 = DummyOperator(task_id='run_this_3')
            run_this_3.set_upstream(run_this_2)
        dr = dag_maker.create_dagrun()
        
        assert dr is not None
        execution_date = dr.execution_date
        running_dates = dag1.get_active_runs()
        try:
            running_date = running_dates[0]
        except Exception:
            running_date = 'Except'
        assert execution_date == running_date, 'Running Date must match Execution Date'
    def test_list_py_file_paths(self):