tests: Explicit registering test tasks step for the swh_app

Prior to this commit, the celery "app" import changed. Making the runtime
application load prior to the tests "swh_app". In effect, making the tasks not
being consumed by workers.

This explicitely forces the tests tasks registering to "swh_app". In effect
clarifying code and fixing the current tests.

Related D2082
Related 8eafc70