**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 = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        scheduler = SchedulerJob(executor=self.null_exec)
        scheduler.processor_agent = mock.MagicMock()
        
        dr = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=DEFAULT_DATE,
            state=State.RUNNING,
        )
        scheduler._schedule_dag_run(dr, {}, session)
        dr = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=dag.following_schedule(dr.execution_date),
            state=State.RUNNING,
        )
        scheduler._schedule_dag_run(dr, {}, session)
        task_instances_list = scheduler._executable_task_instances_to_queued(max_tis=32, session=session)
        assert len(task_instances_list) == 1
    def test_scheduler_verify_pool_full_2_slots_per_task(self):
        """
        Test task instances not queued when pool is full.
        Variation with non-default pool_slots
        """
        dag = DAG(dag_id='test_scheduler_verify_pool_full_2_slots_per_task', start_date=DEFAULT_DATE)
        BashOperator(
            task_id='dummy',
            dag=dag,
            owner='airflow',
            pool='test_scheduler_verify_pool_full_2_slots_per_task',
            pool_slots=2,