Ensure test-resources folder always exist before running tests

Solves issue #60 
Because Application projects generate APKs, which means they generate resources folder as part of the build. but libraries do not generate resources because they don't generate APKs. if there are no resources generated, then we can't copy any resources and then robolectric won't find any resources during tests.

My proposed change is to make the method in `VariantWrapper` abstract and implement it in the `LibraryVariantWrapper` and `ApplicationVariantWrapper` classes to return a different task each.