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
ti1.xcom_push(key='foo', value='bar')
        
        task2 = DummyOperator(task_id='test_xcom_2', dag=dag, owner='airflow')
        ti2 = TI(task=task2, execution_date=exec_date)
        ti2.xcom_push(key='foo', value='baz')
        
        result = ti1.xcom_pull()
        assert result is None
        
        result = ti1.xcom_pull(key='foo')
        assert result in 'baz'
        
        result = ti1.xcom_pull(task_ids='test_xcom_1', key='foo')
        assert result == 'bar'
        
        result = ti1.xcom_pull(task_ids='test_xcom_2', key='foo')
        assert result == 'baz'
        
        result = ti1.xcom_pull(task_ids=['test_xcom_1', 'test_xcom_2'], key='foo')
        assert result == ['bar', 'baz']
    def test_xcom_pull_after_success(self):
        """
        tests xcom set/clear relative to a task in a 'success' rerun scenario
        """
        key = 'xcom_key'
        value = 'xcom_value'
        dag = models.DAG(dag_id='test_xcom', schedule_interval='@monthly')
        task = DummyOperator(
            task_id='test_xcom',
            dag=dag,