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.
Fixes#2997. Interface is optional DSL stanza `container_type`.
This should only be used in rare instances. It is needed for the
case of a naked executable which should not be unpacked.
