### Body

Add unit test for changes in https://github.com/apache/airflow/pull/17305

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
tests/jobs/test_backfill_job.py
import pytest
from airflow import settings
from airflow.cli import cli_parser
from airflow.exceptions import (
    AirflowException,
    AirflowTaskTimeout,
    BackfillUnfinished,
    DagConcurrencyLimitReached,
    NoAvailablePoolSlot,
    TaskConcurrencyLimitReached,
)
from airflow.jobs.backfill_job import BackfillJob
from airflow.models import DagBag, Pool, TaskInstance as TI
from airflow.models.dagrun import DagRun
from airflow.models.taskinstance import TaskInstanceKey
from airflow.operators.dummy import DummyOperator
from airflow.utils import timezone
from airflow.utils.session import create_session
from airflow.utils.state import State
from airflow.utils.timeout import timeout
from airflow.utils.types import DagRunType
from tests.test_utils.db import clear_db_dags, clear_db_pools, clear_db_runs, set_default_pool_slots
from tests.test_utils.mock_executor import MockExecutor
from tests.test_utils.timetables import cron_timetable
logger = logging.getLogger(__name__)
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
@pytest.fixture(scope="module")
def dag_bag():
    return DagBag(include_examples=True)
class TestBackfillJob: