**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
"""
        Test that when on_success_callback is passed to the DAG, has_on_success_callback is stored
        in Serialized JSON blob. And when it is de-serialized dag.has_on_success_callback is set to True.
        When the callback is not set, has_on_success_callback should not be stored in Serialized blob
        and so default to False on de-serialization
        """
        dag = DAG(dag_id='test_dag_on_success_callback_roundtrip', **passed_success_callback)
        BaseOperator(task_id='simple_task', dag=dag, start_date=datetime(2019, 8, 1))
        serialized_dag = SerializedDAG.to_dict(dag)
        if expected_value:
            assert "has_on_success_callback" in serialized_dag["dag"]
        else:
            assert "has_on_success_callback" not in serialized_dag["dag"]
        deserialized_dag = SerializedDAG.from_dict(serialized_dag)
        assert deserialized_dag.has_on_success_callback is expected_value
    @parameterized.expand(
        [
            ({"on_failure_callback": lambda x: print("hi")}, True),
            ({}, False),
        ]
    )
    def test_dag_on_failure_callback_roundtrip(self, passed_failure_callback, expected_value):