**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
dag_model = session.query(DagModel).get(dag.dag_id)
        assert dag_model.next_dagrun == DEFAULT_DATE
        job = SchedulerJob(subdir=os.devnull)
        job.executor = MockExecutor(do_update=False)
        job.processor_agent = mock.MagicMock(spec=DagFileProcessorAgent)
        
        
        job._do_scheduling(session)
        dr1 = dag.get_dagrun(DEFAULT_DATE, session)
        assert dr1 is not None
        assert dr1.state == State.RUNNING
        
        dag_model = session.query(DagModel).get(dag.dag_id)
        assert dag_model.next_dagrun == DEFAULT_DATE + timedelta(minutes=1)
        
        dr = dag.create_dagrun(
            state=State.RUNNING,
            execution_date=timezone.utcnow(),
            run_type=DagRunType.MANUAL,
            session=session,
            external_trigger=True,
        )
        assert dr is not None
        
        DAG.bulk_write_to_db([dag], session)
        
        
        dag_model = session.query(DagModel).get(dag.dag_id)
        assert dag_model.next_dagrun == DEFAULT_DATE + timedelta(minutes=1)
    def test_scheduler_create_dag_runs_check_existing_run(self):