**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
dagbag = DagBag(dag_folder="/dev/null", include_examples=True, read_dags_from_db=False)
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock.MagicMock())
        with create_session() as session:
            session.query(TaskInstance).delete()
            dag = dagbag.get_dag('example_branch_operator')
            task = dag.get_task(task_id='run_this_first')
            ti = TaskInstance(task, DEFAULT_DATE, State.RUNNING)
            session.add(ti)
            session.commit()
            requests = [
                TaskCallbackRequest(
                    full_filepath="A", simple_task_instance=SimpleTaskInstance(ti), msg="Message"
                )
            ]
            dag_file_processor.execute_callbacks(dagbag, requests)
            mock_ti_handle_failure.assert_called_once_with(
                error="Message",
                test_mode=conf.getboolean('core', 'unit_test_mode'),
            )
    def test_process_file_should_failure_callback(self):