### 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.
airflow/jobs/backfill_job.py
"""
        Internal status of the backfill job. This class is intended to be instantiated
        only within a BackfillJob instance and will track the execution of tasks,
        e.g. running, skipped, succeeded, failed, etc. Information about the dag runs
        related to the backfill job are also being tracked in this structure,
        .e.g finished runs, etc. Any other status related information related to the
        execution of dag runs / tasks can be included in this structure since it makes
        it easier to pass it around.
        :param to_run: Tasks to run in the backfill
        :type to_run: dict[tuple[TaskInstanceKey], airflow.models.TaskInstance]
        :param running: Maps running task instance key to task instance object
        :type running: dict[tuple[TaskInstanceKey], airflow.models.TaskInstance]
        :param skipped: Tasks that have been skipped
        :type skipped: set[tuple[TaskInstanceKey]]
        :param succeeded: Tasks that have succeeded so far
        :type succeeded: set[tuple[TaskInstanceKey]]
        :param failed: Tasks that have failed
        :type failed: set[tuple[TaskInstanceKey]]
        :param not_ready: Tasks not ready for execution
        :type not_ready: set[tuple[TaskInstanceKey]]
        :param deadlocked: Deadlocked tasks
        :type deadlocked: set[airflow.models.TaskInstance]
        :param active_runs: Active dag runs at a certain point in time
        :type active_runs: list[DagRun]
        :param executed_dag_run_dates: Datetime objects for the executed dag runs
        :type executed_dag_run_dates: set[datetime.datetime]
        :param finished_runs: Number of finished runs so far
        :type finished_runs: int