Fix DB utils for Airflow Backwards compatability tests - import not existing

[jscheffl]

Fix broken main/canary in https://github.com/apache/airflow/actions/runs/11058948895/job/30726378646
...as of merge #42455

Moving import into function preventing non existing import from utils

ImportError while importing test module '/opt/airflow/tests/providers/google/cloud/log/test_gcs_task_handler.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/providers/google/cloud/log/test_gcs_task_handler.py:31: in <module>
    from tests.test_utils.db import clear_db_dags, clear_db_runs
tests/test_utils/db.py:38: in <module>
    from airflow.models.backfill import Backfill, BackfillDagRun
E   ModuleNotFoundError: No module named 'airflow.models.backfill'