**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
count = scheduler._schedule_dag_run(dr, set(), session)
            assert count == 1
            session.refresh(ti)
            assert ti.state == State.SCHEDULED
    @parameterized.expand(
        [
            [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_depends_on_past(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_depends_on_past',
            start_date=DEFAULT_DATE,
            default_args={
                'depends_on_past': True,
            },