**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
"""
        Verify that all example DAGs work with DAG Serialization by
        checking fields between Serialized Dags & non-Serialized Dags
        """
        fields_to_check = dag.get_serialized_fields() - {
            
            'timezone',
            
            'default_args',
            "_task_group",
        }
        for field in fields_to_check:
            assert getattr(serialized_dag, field) == getattr(
                dag, field
            ), f'{dag.dag_id}.{field} does not match'
        if dag.default_args:
            for k, v in dag.default_args.items():
                if callable(v):
                    
                    assert k in serialized_dag.default_args
                else:
                    assert (
                        v == serialized_dag.default_args[k]
                    ), f'{dag.dag_id}.default_args[{k}] does not match'
        assert serialized_dag.timezone.name == dag.timezone.name
        for task_id in dag.task_ids:
            self.validate_deserialized_task(serialized_dag.get_task(task_id), dag.get_task(task_id))