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
"""
        Test the availability of variables in templates
        """
        Variable.set('a_variable', {'a': {'test': 'value'}}, serialize_json=True)
        with DAG('test-dag', start_date=DEFAULT_DATE):
            task = DummyOperator(task_id='op1')
        ti = TI(task=task, execution_date=DEFAULT_DATE)
        context = ti.get_template_context()
        result = task.render_template(content, context)
        assert result == expected_output
    def test_template_with_json_variable_missing(self):
        with DAG('test-dag', start_date=DEFAULT_DATE):
            task = DummyOperator(task_id='op1')
        ti = TI(task=task, execution_date=DEFAULT_DATE)
        context = ti.get_template_context()
        with pytest.raises(KeyError):
            task.render_template('{{ var.json.get("missing_variable") }}', context)
    def test_execute_callback(self):