**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/serialization/test_dag_serialization.py
"""A serialized DAG can be deserialized in another process."""
        
        
        
        queue = multiprocessing.Queue()
        proc = multiprocessing.Process(target=serialize_subprocess, args=(queue, "airflow/example_dags"))
        proc.daemon = True
        proc.start()
        stringified_dags = {}
        while True:
            v = queue.get()
            if v is None:
                break
            dag = SerializedDAG.from_json(v)
            assert isinstance(dag, DAG)
            stringified_dags[dag.dag_id] = dag
        dags = collect_dags("airflow/example_dags")
        assert set(stringified_dags.keys()) == set(dags.keys())
        
        for dag_id in stringified_dags:
            self.validate_deserialized_dag(stringified_dags[dag_id], dags[dag_id])
    def test_roundtrip_provider_example_dags(self):