**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
self, initial_task_state, expected_task_state
    ):
        session = settings.Session()
        dag_id = 'test_execute_helper_should_change_state_for_tis_without_dagrun'
        dag = DAG(dag_id, start_date=DEFAULT_DATE, default_args={'owner': 'owner1'})
        with dag:
            op1 = DummyOperator(task_id='op1')
        
        dagbag = DagBag(dag_folder="/dev/null", include_examples=False, read_dags_from_db=False)
        dagbag.bag_dag(dag, root_dag=dag)
        dagbag.sync_to_db()
        dag = DagBag(read_dags_from_db=True, include_examples=False).get_dag(dag_id)
        
        dag.clear()
        dr = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,