### 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
dag_id = 'SchedulerJobTest.test_execute_task_instances_is_paused_wont_execute'
        task_id_1 = 'dummy_task'
        with dag_maker(dag_id=dag_id, session=session) as dag:
            DummyOperator(task_id=task_id_1)
        assert isinstance(dag, SerializedDAG)
        self.scheduler_job = SchedulerJob(subdir=os.devnull)
        dr1 = dag_maker.create_dagrun(run_type=DagRunType.BACKFILL_JOB)
        (ti1,) = dr1.task_instances
        ti1.state = State.SCHEDULED
        self.scheduler_job._critical_section_execute_task_instances(session)
        ti1.refresh_from_db(session=session)
        assert State.SCHEDULED == ti1.state
        session.rollback()
    def test_execute_task_instances_backfill_tasks_wont_execute(self, dag_maker):
        """
        Tests that backfill tasks won't get executed.
        """
        dag_id = 'SchedulerJobTest.test_execute_task_instances_backfill_tasks_wont_execute'
        task_id_1 = 'dummy_task'
        with dag_maker(dag_id=dag_id):
            task1 = DummyOperator(task_id=task_id_1)
        self.scheduler_job = SchedulerJob(subdir=os.devnull)
        session = settings.Session()
        dr1 = dag_maker.create_dagrun(run_type=DagRunType.BACKFILL_JOB)
        ti1 = TaskInstance(task1, run_id=dr1.run_id)
        ti1.refresh_from_db()
        ti1.state = State.SCHEDULED
        session.merge(ti1)
        session.flush()