Test casks are already tested covered brew style.

Library/Homebrew/cask/cmd/style.rb

@@ -32,20 +32,13 @@ module Cask
 
       def cask_paths
         @cask_paths ||= if args.named.empty?
-          Tap.map(&:cask_dir).select(&:directory?).concat(test_cask_paths)
+          Tap.map(&:cask_dir).select(&:directory?)
         elsif args.named.any? { |file| File.exist?(file) }
           args.named.map { |path| Pathname(path).expand_path }
         else
           casks.map(&:sourcefile_path)
         end
       end
-
-      def test_cask_paths
-        [
-          Pathname.new("#{HOMEBREW_LIBRARY}/Homebrew/test/support/fixtures/cask/Casks"),
-          Pathname.new("#{HOMEBREW_LIBRARY}/Homebrew/test/support/fixtures/third-party/Casks"),
-        ]
-      end
     end
   end
 end

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

@@ -46,8 +46,6 @@ describe Cask::Cmd::Style, :cask do
         expect(subject).to contain_exactly(
           a_path_ending_with("/homebrew/homebrew-cask/Casks"),
           a_path_ending_with("/third-party/homebrew-tap/Casks"),
-          a_path_ending_with("/Homebrew/test/support/fixtures/cask/Casks"),
-          a_path_ending_with("/Homebrew/test/support/fixtures/third-party/Casks"),
         )
       }
     end