Use consistent naming for per service usage test
Use consistent naming for per service usage test

This is consistent with the name for the monthly version of this
test [^1]. Having consistent test names makes it easier to check
the overall coverage of these functions.

[^1]: https://github.com/alphagov/notifications-api/blob/ab9c2be813aa885bef201fded6012923dd050fd2/tests/app/dao/test_fact_billing_dao.py#L891

alphagov/notifications-api,alphagov/notifications-api