**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
"""Collects DAGs to test."""
    dags = {}
    dags.update(make_simple_dag())
    dags.update(make_user_defined_macro_filter_dag())
    if dag_folder:
        if isinstance(dag_folder, (list, tuple)):
            patterns = dag_folder
        else:
            patterns = [dag_folder]
    else:
        patterns = [
            "airflow/example_dags",
            "airflow/providers/*/example_dags",
            "airflow/providers/*/*/example_dags",
        ]
    for pattern in patterns:
        for directory in glob(f"{ROOT_FOLDER}/{pattern}"):
            dags.update(make_example_dags(directory))
    
    dags = {dag_id: dag for dag_id, dag in dags.items() if not dag.is_subdag}
    return dags
def serialize_subprocess(queue, dag_folder):