**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
"_is_dummy": False,
                "_outlets": [],
                "ui_color": "#f0ede4",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command', 'env'],
                "template_fields_renderers": {'bash_command': 'bash', 'env': 'json'},
                "bash_command": "echo {{ task.task_id }}",
                'label': 'bash_task',
                "_task_type": "BashOperator",
                "_task_module": "airflow.operators.bash",
                "pool": "default_pool",
                "executor_config": {
                    '__type': 'dict',
                    '__var': {
                        "pod_override": {
                            '__type': 'k8s.V1Pod',
                            '__var': PodGenerator.serialize_pod(executor_config_pod),
                        }
                    },
                },
            },
            {
                "task_id": "custom_task",
                "retries": 1,
                "retry_delay": 300.0,
                "_downstream_task_ids": [],
                "_inlets": [],
                "_is_dummy": False,
                "_outlets": [],
                "_operator_extra_links": [{"tests.test_utils.mock_operators.CustomOpLink": {}}],