Port backend unit tests from scalac

[smarter]

There's a lot of tests here: https://github.com/scala/scala/tree/2.12.x/test/junit/scala/tools/nsc/backend/jvm, would be nice if we ported some of them to dotty, here's what we have right now: https://github.com/lampepfl/dotty/tree/master/compiler/test/dotty/tools/backend/jvm