**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_file = os.path.join(
            os.path.dirname(os.path.realpath(__file__)), '../dags/test_on_failure_callback.py'
        )
        dagbag = DagBag(dag_folder=dag_file, include_examples=False)
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock.MagicMock())
        with create_session() as session, NamedTemporaryFile(delete=False) as callback_file:
            session.query(TaskInstance).delete()
            dag = dagbag.get_dag('test_om_failure_callback_dag')
            task = dag.get_task(task_id='test_om_failure_callback_task')
            ti = TaskInstance(task, DEFAULT_DATE, State.RUNNING)
            session.add(ti)
            session.commit()
            requests = [
                TaskCallbackRequest(
                    full_filepath=dag.full_filepath,
                    simple_task_instance=SimpleTaskInstance(ti),
                    msg="Message",
                )
            ]
            callback_file.close()
            with mock.patch.dict("os.environ", {"AIRFLOW_CALLBACK_FILE": callback_file.name}):
                dag_file_processor.process_file(dag_file, requests)
            with open(callback_file.name) as callback_file2:
                content = callback_file2.read()
            assert "Callback fired" == content
            os.remove(callback_file.name)
    def test_should_mark_dummy_task_as_success(self):