**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_id = 'SchedulerJobTest.test_nonexistent_pool'
        task_id = 'dummy_wrong_pool'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, concurrency=16)
        task = DummyOperator(dag=dag, task_id=task_id, pool="this_pool_doesnt_exist")
        dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        scheduler = SchedulerJob(subdir=os.devnull)
        session = settings.Session()
        dag_model = DagModel(
            dag_id=dag_id,
            is_paused=False,
            concurrency=dag.concurrency,
            has_task_concurrency_limits=False,
        )
        session.add(dag_model)
        dr = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=DEFAULT_DATE,
            state=State.RUNNING,
        )
        ti = TaskInstance(task, dr.execution_date)
        ti.state = State.SCHEDULED
        session.merge(ti)
        session.commit()
        res = scheduler._executable_task_instances_to_queued(max_tis=32, session=session)
        session.flush()
        assert 0 == len(res)
        session.rollback()
    def test_find_executable_task_instances_none(self):