Test sanity checks by calling top-level task
Test sanity checks by calling top-level task

Since the checks are only performed in one place we can easily take
extra care to ensure this in the tests, noting that we don't need to
do any additional setup, except if no exception is raised - I've left
these tests as-is, to avoid doing more setup.

Note that we still check the happy path for when a provider message is
already sending - just in a different test [1].

[1]: https://github.com/alphagov/notifications-api/blob/3d71815956eb930a1683d47e60d8c4a53974fd32/tests/app/celery/test_broadcast_message_tasks.py#L263

alphagov/notifications-api,alphagov/notifications-api