**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
"""Validate pickle in a subprocess."""
    dags = collect_dags(dag_folder)
    for dag in dags.values():
        queue.put(SerializedDAG.to_json(dag))
    queue.put(None)
class TestStringifiedDAGs(unittest.TestCase):
    """Unit tests for stringified DAGs."""
    def setUp(self):
        super().setUp()
        BaseHook.get_connection = mock.Mock(
            return_value=Connection(
                extra=(
                    '{'
                    '"project_id": "mock", '
                    '"location": "mock", '
                    '"instance": "mock", '
                    '"database_type": "postgres", '
                    '"use_proxy": "False", '
                    '"use_ssl": "False"'
                    '}'
                )
            )
        )
        self.maxDiff = None  
    def test_serialization(self):