Refactor supplementary test code

https://eaflood.atlassian.net/browse/WATER-3854

We are about to [Update "create bill run" endpoint to create a bill run](#56). Currently, everything is happening in the controller whereas our convention is to do everything in a service and keep our controllers ['skinny'](https://blog.makersacademy.com/forget-fat-models-its-time-for-skinny-controllers-and-skinny-models-a9b84ec481b7).

So, we need a service, something like `SupplementaryService`. But oh no! It already exists.

This change is about refactoring the code currently in `SupplementaryService` to a more explicit testing version so we can free up the namespace. While we're at it, we also rename our `/test` path to `/qa` because everyone is getting confused when they come across a `test/` folder in the repo!