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.
Merge pull request #7430 from rolandwalker/new_header_test_casks

remove old-style header line from test Casks