support stub the return type of void

Thanks for contributing !

I think that adding a stub statement for a method that returns nothing, won't make sense without adding also a matching statement to verify() - which is currently a missing feature that requires more work...
you're technically right - fields defined in the test class should technically be defined with private access modifier. but...practically, a unit test class would not be referenced and reused by other classes (expect in test inheritance). there no practical motivation to hide internal implementation. so it's better to minimize the test code and make it easily readable.

wrdv / testme-idea