remove old-style header line from test Casks

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.  Since those tests don't read from a separate Cask file, they don't block removal of the legacy form on loads.