**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
date = dag.following_schedule(date)
        dr2 = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=date,
            state=State.RUNNING,
        )
        date = dag.following_schedule(date)
        dr3 = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=date,
            state=State.RUNNING,
        )
        
        assert dr1 is not None
        assert dr2 is not None
        assert dr3 is not None
        assert len(DagRun.find(dag_id=dag.dag_id, state=State.RUNNING, session=session)) == 3
        
        dag.max_active_runs = 1
        
        
        with create_session() as session:
            num_scheduled = scheduler._schedule_dag_run(dr1, set(), session)
            assert num_scheduled == 1
            num_scheduled = scheduler._schedule_dag_run(dr2, {dr1.execution_date}, session)
            assert num_scheduled == 0
            num_scheduled = scheduler._schedule_dag_run(dr3, {dr1.execution_date}, session)
            assert num_scheduled == 0
    @patch.object(TaskInstance, 'handle_failure_with_callback')
    def test_execute_on_failure_callbacks(self, mock_ti_handle_failure):