Generator tests

Addresses https://github.com/ArcBees/GWTP/issues/99 and provides first tests for `ApplicationControllerGenerator` and `GinjectorGenerator` with multiple gin modules as well as bootstrappers and pre-bootstrappers.
Also fixes a refactoring bug with pre-bootstrappers and optimizes usage of multi-valued configuration properties. Both discovered while writing the test.

Still to do are tests for ginjector extensions, ginjector with sync/async presenters and presenter bundles.