### 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
"""
        [JIRA-1357] Test the 'list_py_file_paths' function used by the
        scheduler to list and load DAGs.
        """
        detected_files = set()
        expected_files = set()
        
        ignored_files = {
            'no_dags.py',
            'test_invalid_cron.py',
            'test_zip_invalid_cron.zip',
            'test_ignore_this.py',
        }
        for root, _, files in os.walk(TEST_DAG_FOLDER):
            for file_name in files:
                if file_name.endswith('.py') or file_name.endswith('.zip'):
                    if file_name not in ignored_files:
                        expected_files.add(f'{root}/{file_name}')