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
"""
        Test that ignore_first_depends_on_past doesn't affect results
        (this is the same test as
        test_dagrun_deadlock_ignore_depends_on_past_advance_ex_date except
        that start_date == execution_date so depends_on_past is irrelevant).
        """
        self.evaluate_dagrun(
            dag_id='test_dagrun_states_deadlock',
            expected_task_states={
                'test_depends_on_past': State.SUCCESS,
                'test_depends_on_past_2': State.SUCCESS,
            },
            dagrun_state=State.SUCCESS,
            run_kwargs=dict(ignore_first_depends_on_past=True))
    def test_scheduler_start_date(self):