### 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 that scheduler._create_dag_runs does not raise an error when the DAG does not exist
        in serialized_dag table
        """
        with dag_maker(dag_id='test_scheduler_create_dag_runs_does_not_raise_error', serialized=False):
            DummyOperator(
                task_id='dummy',
            )
        self.scheduler_job = SchedulerJob(subdir=os.devnull, executor=self.null_exec)
        self.scheduler_job.processor_agent = mock.MagicMock()
        caplog.set_level('FATAL')
        caplog.clear()
        with create_session() as session, caplog.at_level(
            'ERROR',
            logger='airflow.jobs.scheduler_job',
        ):
            self.scheduler_job._create_dag_runs([dag_maker.dag_model], session)
            assert caplog.messages == [
                "DAG 'test_scheduler_create_dag_runs_does_not_raise_error' not found in serialized_dag table",
            ]
    def test_bulk_write_to_db_external_trigger_dont_skip_scheduled_run(self, dag_maker):