**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
"ui_color": "#fff",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command'],
                "template_fields_renderers": {},
                "_task_type": "CustomOperator",
                "_task_module": "tests.test_utils.mock_operators",
                "pool": "default_pool",
                'label': 'custom_task',
            },
        ],
        "timezone": "UTC",
        "_access_control": {
            "__type": "dict",
            "__var": {
                "test_role": {
                    "__type": "set",
                    "__var": [permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT],
                }
            },
        },
    },
}
ROOT_FOLDER = os.path.realpath(
    os.path.join(os.path.dirname(os.path.realpath(__file__)), os.pardir, os.pardir)
)
def make_example_dags(module_path):
    """Loads DAGs from a module for test."""
    dagbag = DagBag(module_path)
    return dagbag.dags
def make_simple_dag():