pytest_plugin: Explicitly set hostname in broker_url for celery TestApp

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

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Migrated from D5776 (view on Phabricator)