**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
} == {(ti.task_id, ti.state) for ti in tis}
        for state, start_date, end_date, duration in [
            (ti.state, ti.start_date, ti.end_date, ti.duration) for ti in tis
        ]:
            if state == 'success':
                assert start_date is not None
                assert end_date is not None
                assert 0.0 == duration
            else:
                assert start_date is None
                assert end_date is None
                assert duration is None
        scheduler_job._schedule_dag_run(dr, {}, session)
        with create_session() as session:
            tis = session.query(TaskInstance).all()
        assert 5 == len(tis)
        assert {
            ('test_task_a', 'success'),
            ('test_task_b', 'success'),
            ('test_task_c', 'success'),
            ('test_task_on_execute', 'scheduled'),
            ('test_task_on_success', 'scheduled'),
        } == {(ti.task_id, ti.state) for ti in tis}
        for state, start_date, end_date, duration in [
            (ti.state, ti.start_date, ti.end_date, ti.duration) for ti in tis
        ]:
            if state == 'success':
                assert start_date is not None
                assert end_date is not None
                assert 0.0 == duration