Port a core set of unit tests to kotlin

A number of issues mention kotlin: https://github.com/google/truth/issues?q=is%3Aissue+is%3Aopen+kotlin

Before worrying about any of them I think we should have a core set of unit tests ported to Kotlin. These only need to cover varying _API shapes_, not all variations of behavior. (I'd put this requirement on any library that wants to claim it's for both languages.) Of course, a custom subject or two should be included.

(I might actually be able to take this task, but not promising yet.)