**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
try:
            dags_folder = mkdtemp()
            filename_to_parse = os.path.join(dags_folder, TEMP_DAG_FILENAME)
            
            with open(filename_to_parse, 'w') as file_to_parse:
                file_to_parse.writelines(UNPARSEABLE_DAG_FILE_CONTENTS)
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
        finally:
            shutil.rmtree(dags_folder)
        
        self.run_single_scheduler_loop_with_no_dags(dags_folder)
        with create_session() as session:
            import_errors = session.query(errors.ImportError).all()
        assert len(import_errors) == 0
    def test_import_error_tracebacks(self):