**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

tests/jobs/test_scheduler_job.py
"""
        Test that externally triggered Dag Runs should not affect (by skipping) next
        scheduled DAG runs
        """
        dag = DAG(
            dag_id='test_bulk_write_to_db_external_trigger_dont_skip_scheduled_run',
            start_date=DEFAULT_DATE,
            schedule_interval="*/1 * * * *",
            max_active_runs=5,
            catchup=True,
        )
        DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        session = settings.Session()
        dag.clear()
        dagbag = DagBag(
            dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"),
            include_examples=False,
            read_dags_from_db=True,
        )
        dagbag.bag_dag(dag=dag, root_dag=dag)
        
        dagbag.sync_to_db(session)
        dag = dagbag.get_dag(dag.dag_id)