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.
I do not get much this line. I think that it is intended for guys who want to learn something or is pointed here because they do something wrong. Almost all good guide lines I see contain examples in rules or how section, not separated as it is much harder to link it together. see e.g. https://github.com/reachlocal/rspec-style-guide or betterspecs you mention have ilustration code below each statement, so it is easier to demonstrate what you mean