**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
dags_folder = mkdtemp()
        try:
            unparseable_filename = os.path.join(dags_folder, TEMP_DAG_FILENAME)
            with open(unparseable_filename, "w") as unparseable_file:
                unparseable_file.writelines(INVALID_DAG_WITH_DEPTH_FILE_CONTENTS)
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
        finally:
            shutil.rmtree(dags_folder)
        with create_session() as session:
            import_errors = session.query(errors.ImportError).all()
        assert len(import_errors) == 1
        import_error = import_errors[0]
        assert import_error.filename == unparseable_filename
        expected_stacktrace = (
            "Traceback (most recent call last):\n"
            '  File "{}", line 3, in <module>\n'
            "    something()\n"
            '  File "{}", line 2, in something\n'
            "    return airflow_DAG\n"
            "NameError: name 'airflow_DAG' is not defined\n"
        )
        assert import_error.stacktrace == expected_stacktrace.format(
            unparseable_filename, unparseable_filename
        )
    @conf_vars({("core", "dagbag_import_error_traceback_depth"): "1"})
    def test_import_error_traceback_depth(self):