If you have a DAG with `max_active_runs=1` and it is running some tasks, and then you manually trigger another run of this DAG, both runs end up not scheduling anything and you see this message over and over in the logs:

```
[2020-10-16 14:00:34,118]  2437572 {{airflow.jobs.scheduler_job.SchedulerJob scheduler_job.py:1657}} INFO - DAG scenario1_case2_01_1 already has 1 active runs, not queuing any more tasks
```

A longer term fix for this would be to introduce a "queued" state for DagRuns, and then when manually creating dag runs set it to queued, and only have the scheduler set DagRuns to running.

That may or may not be less work than fixing the current mechanism.
tests/jobs/test_scheduler_job.py
"""
        Utility function that runs a single scheduler loop without actually
        changing/scheduling any dags. This is useful to simulate the other side effects of
        running a scheduler loop, e.g. to see what parse errors there are in the
        dags_folder.
        :param dags_folder: the directory to traverse
        :type dags_folder: str
        """
        scheduler = SchedulerJob(
            executor=self.null_exec,
            num_times_parse_dags=1,
            subdir=os.path.join(dags_folder))
        scheduler.heartrate = 0
        scheduler.run()
    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)
        
        current_children = set(current_process.children(recursive=True)) - set(
            old_children)
        self.assertFalse(current_children)
    @mock.patch('airflow.jobs.scheduler_job.TaskCallbackRequest')
    @mock.patch('airflow.jobs.scheduler_job.Stats.incr')
    def test_process_executor_events(self, mock_stats_incr, mock_task_callback):