Merge thirtyinch-test in thirtyinch

[StefMa]

Currently we have a extra thirtyinch-test dependency which includes just a single file.
Because it is way more overhead to maintain these extra package we agreed that we want to merge it into the "main" thirtyinch module.

We want to have a similar idea like RxJava in tests.
Just calling:

val presenterInstructor = MyPresenterImpl().test()

Currently we are not really sure if we really want to return the PresenterInstructor or another (helper) object which hold the real presenter and the instructor...
Maybe later we want to add more functionality to it which can nicely delegate the "actions" (or maybe asserts 🤔 ) to the correct object...