**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
[
            [State.NONE, None, None],
            [
                State.UP_FOR_RETRY,
                timezone.utcnow() - datetime.timedelta(minutes=30),
                timezone.utcnow() - datetime.timedelta(minutes=15),
            ],
            [
                State.UP_FOR_RESCHEDULE,
                timezone.utcnow() - datetime.timedelta(minutes=30),
                timezone.utcnow() - datetime.timedelta(minutes=15),
            ],
        ]
    )
    def test_dag_file_processor_process_task_instances(self, state, start_date, end_date):
        """
        Test if _process_task_instances puts the right task instances into the
        mock_list.
        """
        dag = DAG(dag_id='test_scheduler_process_execute_task', start_date=DEFAULT_DATE)
        BashOperator(task_id='dummy', dag=dag, owner='airflow', bash_command='echo hi')
        with create_session() as session:
            orm_dag = DagModel(dag_id=dag.dag_id)
            session.merge(orm_dag)
        dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        scheduler = SchedulerJob(subdir=os.devnull)
        scheduler.processor_agent = mock.MagicMock()
        scheduler.dagbag.bag_dag(dag, root_dag=dag)
        dag.clear()
        dr = dag.create_dagrun(