Since the release of `kombu 5.1.0`, a warning is now issued when a hostname
is not set in the `broker_url` config value of a celery app.
That change makes the test_celery_monitor_ping test fails due to that
new unexpected warning.
So explicitly add localhost hostname in the `broker_url` value of the
celery TestApp config.
Related Github commit and issue:
- https://github.com/celery/kombu/commit/c7068ef240f8911f3e843b305ed2c58d7a57f6d6
- https://github.com/celery/celery/issues/6661