**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
empty_dir = mkdtemp()
        current_process = psutil.Process()
        old_children = current_process.children(recursive=True)
        scheduler = SchedulerJob(subdir=empty_dir, num_runs=1, executor=MockExecutor(do_update=False))
        scheduler.run()
        shutil.rmtree(empty_dir)
        
        current_children = set(current_process.children(recursive=True)) - set(old_children)
        assert not current_children
    @mock.patch('airflow.jobs.scheduler_job.TaskCallbackRequest')
    @mock.patch('airflow.jobs.scheduler_job.Stats.incr')
    def test_process_executor_events(self, mock_stats_incr, mock_task_callback):
        dag_id = "test_process_executor_events"
        dag_id2 = "test_process_executor_events_2"
        task_id_1 = 'dummy_task'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, full_filepath="/test_path1/")
        dag2 = DAG(dag_id=dag_id2, start_date=DEFAULT_DATE, full_filepath="/test_path1/")
        task1 = DummyOperator(dag=dag, task_id=task_id_1)
        DummyOperator(dag=dag2, task_id=task_id_1)
        dag.fileloc = "/test_path1/"
        dag2.fileloc = "/test_path1/"
        executor = MockExecutor(do_update=False)
        task_callback = mock.MagicMock()
        mock_task_callback.return_value = task_callback
        scheduler = SchedulerJob(executor=executor)
        scheduler.processor_agent = mock.MagicMock()
        session = settings.Session()
        dag.sync_to_db(session=session)