Add missing `:cask` metadata to specs.

Library/Homebrew/test/cask/cli/doctor_spec.rb

Library/Homebrew/test/cask/cli/style_spec.rb

Library/Homebrew/test/cask/dsl/stanza_proxy_spec.rb

Library/Homebrew/test/cask/dsl/version_spec.rb

Library/Homebrew/test/cask/macos_spec.rb

Library/Homebrew/test/cask/verify/checksum_spec.rb