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
import psutil
import pytest
from parameterized import parameterized
from sqlalchemy import func
from sqlalchemy.exc import OperationalError
import airflow.example_dags
import airflow.smart_sensor_dags
from airflow import settings
from airflow.configuration import conf
from airflow.exceptions import AirflowException
from airflow.executors.base_executor import BaseExecutor
from airflow.jobs.backfill_job import BackfillJob
from airflow.jobs.scheduler_job import DagFileProcessor, SchedulerJob
from airflow.models import DAG, DagBag, DagModel, Pool, SlaMiss, TaskInstance, errors
from airflow.models.dagrun import DagRun
from airflow.models.serialized_dag import SerializedDagModel
from airflow.models.taskinstance import SimpleTaskInstance, TaskInstanceKey
from airflow.operators.bash import BashOperator
from airflow.operators.dummy_operator import DummyOperator
from airflow.serialization.serialized_objects import SerializedDAG
from airflow.utils import timezone
from airflow.utils.callback_requests import DagCallbackRequest, TaskCallbackRequest
from airflow.utils.dag_processing import DagFileProcessorAgent
from airflow.utils.dates import days_ago
from airflow.utils.file import list_py_file_paths
from airflow.utils.session import create_session, provide_session
from airflow.utils.state import State
from airflow.utils.types import DagRunType
from tests.test_utils.asserts import assert_queries_count
from tests.test_utils.config import conf_vars, env_vars