Testing helpers should be moved to own project too #375

There are a lot of helper functions in custom-generator and processor projects tests (maybe even in others). These help to check the result of generated classes. The goal is to find generalized functions or to create base functions which could be collected in a separate project which could be used on test scope. It should hold all needed test-dependencies too. These should then be consumed transitive on test scope by the other projects which need them.