**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
clear_db_runs()
        clear_db_pools()
        clear_db_dags()
        clear_db_sla_miss()
        clear_db_errors()
        clear_db_jobs()
        clear_db_serialized_dags()
    def setUp(self):
        self.clean_db()
        
        
        self.null_exec = MockExecutor()
    def tearDown(self) -> None:
        self.clean_db()
    def create_test_dag(self, start_date=DEFAULT_DATE, end_date=DEFAULT_DATE + timedelta(hours=1), **kwargs):
        dag = DAG(
            dag_id='test_scheduler_reschedule',
            start_date=start_date,
            
            end_date=end_date,
        )
        dag.clear()
        dag.is_subdag = False
        with create_session() as session:
            orm_dag = DagModel(dag_id=dag.dag_id, is_paused=False)
            session.merge(orm_dag)
            session.commit()
        return dag
    @classmethod
    def setUpClass(cls):