convert MarkDown tests to Cucumber

- Replaced MarkDown test descriptions (taken from Zombie Killer) by Cucumber
- Removed our own MarkDown -> RSpec generator
- Use Cucumber (upstream standard)
- See https://travis-ci.org/yast/rubocop-yast/builds/44026313#L218 for Cucumber output
- SimpleCov merges RSpec and Cucumber code coverage automatically (the total coverage is unchanged)