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.
Replacing `link` for almost all cases.
The `link` stanza can still appear in error messages, because
under the hood, `app` is still implemented as a pure synonym
for the `link` artifact. That will change automatically when
we factor into separate artifacts.
