First draft for "how to write tests" for Yast code

TBD -> either write own examples or maybe rather link those on http://betterspecs.org/

I'm actually expecting a bit more changes here. But to open the discussion, I'm opening this PR.
@jreidinger I've changed that, but it's hard to make you happy as I don't exactly know how to do it :) This line came from our meeting results and I think I haven't written it.