Since the release of `kombu 5.1.0`, a warning is now issued when the
broker URL is not explicitly set in the celery config.
That change makes the `test_celery_monitor_ping` test [fails](https://jenkins.softwareheritage.org/job/DENV/job/tests/1091/artifact/swh-scheduler.log) due to that
new unexpected warning.
So ensure to explicitly set the `broker_url` value it in the celery
TestApp config.
Related Github commit and issue:
- https://github.com/celery/kombu/commit/c7068ef240f8911f3e843b305ed2c58d7a57f6d6
- https://github.com/celery/celery/issues/6661