celery/tests: mostly revert e770eb30 to fix celery app initialization in tests
This revision did fix tests for the scheduler itself, but broke all other tests of scheduler dependent swh packages.
In this fix, we ensure we override the app
in
swh.scheduler.celery_backend.config, since it is used by all
celery task declarations (via the @app.task() decorator).
Migrated from D2161 (view on Phabricator)