**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/jobs/test_scheduler_job.py
for file_path in list_py_file_paths(TEST_DAG_FOLDER, include_examples=False):
            detected_files.add(file_path)
        assert detected_files == expected_files
        ignored_files = {
            'helper.py',
        }
        example_dag_folder = airflow.example_dags.__path__[0]
        for root, _, files in os.walk(example_dag_folder):  
            for file_name in files:
                if file_name.endswith('.py') or file_name.endswith('.zip'):
                    if file_name not in ['__init__.py'] and file_name not in ignored_files:
                        expected_files.add(os.path.join(root, file_name))
        detected_files.clear()
        for file_path in list_py_file_paths(TEST_DAG_FOLDER, include_examples=True):
            detected_files.add(file_path)
        assert detected_files == expected_files
        smart_sensor_dag_folder = airflow.smart_sensor_dags.__path__[0]
        for root, _, files in os.walk(smart_sensor_dag_folder):
            for file_name in files:
                if (file_name.endswith('.py') or file_name.endswith('.zip')) and file_name not in [
                    '__init__.py'
                ]:
                    expected_files.add(os.path.join(root, file_name))
        detected_files.clear()
        for file_path in list_py_file_paths(
            TEST_DAG_FOLDER, include_examples=True, include_smart_sensor=True
        ):
            detected_files.add(file_path)
        assert detected_files == expected_files
    def test_adopt_or_reset_orphaned_tasks_nothing(self):