**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
"""
        Test task instances with higher priority are not queued
        when pool does not have enough slots.
        Though tasks with lower priority might be executed.
        """
        dag = DAG(dag_id='test_scheduler_verify_priority_and_slots', start_date=DEFAULT_DATE)
        
        BashOperator(
            task_id='test_scheduler_verify_priority_and_slots_t0',
            dag=dag,
            owner='airflow',
            pool='test_scheduler_verify_priority_and_slots',
            pool_slots=2,
            priority_weight=2,
            bash_command='echo hi',
        )
        
        BashOperator(
            task_id='test_scheduler_verify_priority_and_slots_t1',
            dag=dag,
            owner='airflow',
            pool='test_scheduler_verify_priority_and_slots',
            pool_slots=1,
            priority_weight=3,
            bash_command='echo hi',
        )
        
        BashOperator(
            task_id='test_scheduler_verify_priority_and_slots_t2',
            dag=dag,