This tests fails occasionally on mysql at least:

https://github.com/apache/airflow/pull/11555/checks?check_run_id=1260840314#step:9:1125

```
=================================== FAILURES ===================================
_____________ TestSchedulerJob.test_no_orphan_process_will_be_left _____________

self = <tests.jobs.test_scheduler_job.TestSchedulerJob testMethod=test_no_orphan_process_will_be_left>

    def test_no_orphan_process_will_be_left(self):
        empty_dir = mkdtemp()
        current_process = psutil.Process()
        old_children = current_process.children(recursive=True)
        scheduler = SchedulerJob(subdir=empty_dir,
                                 num_runs=1,
                                 executor=MockExecutor(do_update=False))
        scheduler.run()
        shutil.rmtree(empty_dir)
    
        # Remove potential noise created by previous tests.
        current_children = set(current_process.children(recursive=True)) - set(
            old_children)
>       self.assertFalse(current_children)
E       AssertionError: {psutil.Process(pid=3474, name='/usr/local/bin/', status='running', started='19:26:22')} is not false

tests/jobs/test_scheduler_job.py:871: AssertionError
```



tests/jobs/test_scheduler_job.py
clear_db_jobs,
    clear_db_pools,
    clear_db_runs,
    clear_db_serialized_dags,
    clear_db_sla_miss,
    set_default_pool_slots,
)
from tests.test_utils.mock_executor import MockExecutor
from tests.test_utils.mock_operators import CustomOperator
ROOT_FOLDER = os.path.realpath(
    os.path.join(os.path.dirname(os.path.realpath(__file__)), os.pardir, os.pardir)
)
PERF_DAGS_FOLDER = os.path.join(ROOT_FOLDER, "tests", "test_utils", "perf", "dags")
ELASTIC_DAG_FILE = os.path.join(PERF_DAGS_FOLDER, "elastic_dag.py")
TEST_DAG_FOLDER = os.environ['AIRFLOW__CORE__DAGS_FOLDER']
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
TRY_NUMBER = 1
PARSEABLE_DAG_FILE_CONTENTS = '"airflow DAG"'
UNPARSEABLE_DAG_FILE_CONTENTS = 'airflow DAG'
INVALID_DAG_WITH_DEPTH_FILE_CONTENTS = "def something():\n    return airflow_DAG\nsomething()"
TEMP_DAG_FILENAME = "temp_dag.py"
@pytest.fixture(scope="class")
def disable_load_example():
    with conf_vars({('core', 'load_examples'): 'false'}):
        with env_vars({('core', 'load_examples'): 'false'}):
            yield
@pytest.mark.usefixtures("disable_load_example")
class TestDagFileProcessor(unittest.TestCase):
    @staticmethod
    def clean_db():