**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
)
        BashOperator(task_id='dummy1', dag=dag, owner='airflow', bash_command='echo hi')
        BashOperator(task_id='dummy2', dag=dag, owner='airflow', bash_command='echo hi')
        with create_session() as session:
            orm_dag = DagModel(dag_id=dag.dag_id)
            session.merge(orm_dag)
        dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        scheduler = SchedulerJob(subdir=os.devnull)
        scheduler.processor_agent = mock.MagicMock()
        scheduler.dagbag.bag_dag(dag, root_dag=dag)
        dag.clear()
        dr = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=DEFAULT_DATE,
            state=State.RUNNING,
        )
        assert dr is not None
        with create_session() as session:
            tis = dr.get_task_instances(session=session)
            for ti in tis:
                ti.state = state
                ti.start_date = start_date
                ti.end_date = end_date
            count = scheduler._schedule_dag_run(dr, set(), session)
            assert count == 2
            session.refresh(tis[0])
            session.refresh(tis[1])
            assert tis[0].state == State.SCHEDULED
            assert tis[1].state == State.SCHEDULED
    def test_scheduler_job_add_new_task(self):