**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
owner='airflow',
            pool='test_scheduler_verify_priority_and_slots',
            pool_slots=1,
            priority_weight=1,
            bash_command='echo hi',
        )
        dagbag = DagBag(
            dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"),
            include_examples=False,
            read_dags_from_db=True,
        )
        dagbag.bag_dag(dag=dag, root_dag=dag)
        dagbag.sync_to_db()
        session = settings.Session()
        pool = Pool(pool='test_scheduler_verify_priority_and_slots', slots=2)
        session.add(pool)
        session.commit()
        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)
        task_instances_list = scheduler._executable_task_instances_to_queued(max_tis=32, session=session)
        
        assert len(task_instances_list) == 2
        ti0 = (