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
assert 'test_echo_env_variables' == os.environ['AIRFLOW_CTX_DAG_ID']
        assert 'hive_in_python_op' == os.environ['AIRFLOW_CTX_TASK_ID']
        assert DEFAULT_DATE.isoformat() == os.environ['AIRFLOW_CTX_EXECUTION_DATE']
        assert DagRun.generate_run_id(DagRunType.MANUAL, DEFAULT_DATE) == os.environ['AIRFLOW_CTX_DAG_RUN_ID']
    def test_echo_env_variables(self):
        dag = DAG(
            'test_echo_env_variables',
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=10),
        )
        op = PythonOperator(
            task_id='hive_in_python_op', dag=dag, python_callable=self._env_var_check_callback
        )
        dag.create_dagrun(
            run_type=DagRunType.MANUAL,
            execution_date=DEFAULT_DATE,
            start_date=DEFAULT_DATE,
            state=State.RUNNING,
            external_trigger=False,
        )
        ti = TI(task=op, execution_date=DEFAULT_DATE)
        ti.state = State.RUNNING
        session = settings.Session()
        session.merge(ti)
        session.commit()
        ti._run_raw_task()
        ti.refresh_from_db()
        assert ti.state == State.SUCCESS
    @patch.object(Stats, 'incr')
    def test_task_stats(self, stats_mock):