**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
"""Make very simple DAG to verify serialization result."""
    with DAG(
        dag_id='simple_dag',
        default_args={
            "retries": 1,
            "retry_delay": timedelta(minutes=5),
            "depends_on_past": False,
        },
        start_date=datetime(2019, 8, 1),
        is_paused_upon_creation=False,
        access_control={"test_role": {permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT}},
    ) as dag:
        CustomOperator(task_id='custom_task')
        BashOperator(
            task_id='bash_task',
            bash_command='echo {{ task.task_id }}',
            owner='airflow',
            executor_config={"pod_override": executor_config_pod},
        )
        return {'simple_dag': dag}
def make_user_defined_macro_filter_dag():
    """Make DAGs with user defined macros and filters using locally defined methods.
    For Webserver, we do not include ``user_defined_macros`` & ``user_defined_filters``.
    The examples here test:
        (1) functions can be successfully displayed on UI;
        (2) templates with function macros have been rendered before serialization.
    """
    def compute_next_execution_date(dag, execution_date):