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/models/test_taskinstance.py
naive_datetime = DEFAULT_DATE.replace(tzinfo=None)
        op_no_dag = DummyOperator(
            task_id='test_task_naive_datetime', start_date=naive_datetime, end_date=naive_datetime
        )
        assert op_no_dag.start_date.tzinfo
        assert op_no_dag.end_date.tzinfo
    def test_set_dag(self):
        """
        Test assigning Operators to Dags, including deferred assignment
        """
        dag = DAG('dag', start_date=DEFAULT_DATE)
        dag2 = DAG('dag2', start_date=DEFAULT_DATE)
        op = DummyOperator(task_id='op_1', owner='test')
        
        assert not op.has_dag()
        with pytest.raises(AirflowException):
            getattr(op, 'dag')
        
        with pytest.raises(TypeError):
            op.dag = 1
        op.dag = dag
        
        with pytest.raises(AirflowException):
            op.dag = dag2
        
        op.dag = dag
        assert op.dag is dag
        assert op in dag.tasks
    def test_infer_dag(self):