**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)
            ti1 = TaskInstance(task1, dr.execution_date)
            ti2 = TaskInstance(task2, dr.execution_date)
            tis.append(ti1)
            tis.append(ti2)
            ti1.refresh_from_db()
            ti2.refresh_from_db()
            ti1.state = State.SCHEDULED
            ti2.state = State.SCHEDULED
            session.merge(ti1)
            session.merge(ti2)
            session.flush()
        scheduler.max_tis_per_query = 2
        res = scheduler._critical_section_execute_task_instances(session)
        assert 2 == res
        scheduler.max_tis_per_query = 8
        with mock.patch.object(
            type(scheduler.executor), 'slots_available', new_callable=mock.PropertyMock
        ) as mock_slots:
            mock_slots.return_value = 2
            
            assert 2 == res
            res = scheduler._critical_section_execute_task_instances(session)
        res = scheduler._critical_section_execute_task_instances(session)
        assert 4 == res
        for ti in tis:
            ti.refresh_from_db()
            assert State.QUEUED == ti.state
    def test_execute_task_instances_unlimited(self):