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
with DAG(
        dag_id='test_task_with_upstream_skip_dag',
        start_date=DEFAULT_DATE,
        schedule_interval=None
    ) as dag:
        dummy1 = DummyOperator(task_id='dummy1')
        dummy2 = DummyOperator(task_id="dummy2")
        dummy3 = DummyOperator(task_id="dummy3")
        [dummy1, dummy2] >> dummy3
    
    dag.clear()
    dr = dag.create_dagrun(run_type=DagRunType.MANUAL,
                           state=State.RUNNING,
                           execution_date=DEFAULT_DATE)
    assert dr is not None
    with create_session() as session:
        tis = {ti.task_id: ti for ti in dr.get_task_instances(session=session)}
        
        tis[dummy1.task_id].state = State.SKIPPED
        tis[dummy2.task_id].state = State.SUCCESS
        assert tis[dummy3.task_id].state == State.NONE
    
    
    with create_session() as session:
        tis = {ti.task_id: ti for ti in dr.get_task_instances(session=session)}
        assert tis[dummy1.task_id].state == State.SKIPPED
        assert tis[dummy2.task_id].state == State.SUCCESS
        
        assert tis[dummy3.task_id].state == State.SKIPPED
class TestSchedulerJobQueriesCount(unittest.TestCase):