This takes the form of a horrible hack: DSL version numbers may
end with "test", *eg* ":v1test". Such Casks are mapped to class
`TestCask` instead of class `Cask`.
The intention is that all of this logic will be removed when
Casks are migrated away from separate classes.
Tests driven by RSpec are still todo.
Keeping the `link` stanza in the DSL is a mistake: it will be
confusing when combined with the ability to install by copying.
This PR retires `link` completely, and adds a generic artifact
stanza called `artifact`. (`link` is removed from the docs, but
will still work for compatibility during transition.)
This only affects one current Cask (dwarf-fortress.rb), and that
Cask may be changed to use `suite` in the future.
