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
with DAG(dag_id='test_dag'):
            task = DummyOperator(task_id='task', start_date=DEFAULT_DATE)
            downstream_task = DummyOperator(task_id='downstream_task', start_date=DEFAULT_DATE)
            task >> downstream_task
        ti = TI(task, DEFAULT_DATE)
        downstream_ti = TI(downstream_task, DEFAULT_DATE)
        downstream_ti.set_state(downstream_ti_state)
        assert ti.are_dependents_done() == expected_are_dependents_done
    def test_xcom_pull(self):
        """
        Test xcom_pull, using different filtering methods.
        """
        dag = models.DAG(
            dag_id='test_xcom',
            schedule_interval='@monthly',
            start_date=timezone.datetime(2016, 6, 1, 0, 0, 0),
        )
        exec_date = timezone.utcnow()
        
        task1 = DummyOperator(task_id='test_xcom_1', dag=dag, owner='airflow')
        ti1 = TI(task=task1, execution_date=exec_date)