Add test for command help strings.

Library/Homebrew/cask/lib/hbc/cli/base.rb

@@ -14,7 +14,7 @@ module Hbc
       end
 
       def self.help
-        "No help available for the #{command_name} command"
+        nil
       end
 
       def self.needs_init?

Library/Homebrew/cask/lib/hbc/cli/internal_audit_modified_casks.rb

@@ -23,6 +23,10 @@ module Hbc
         args.reject { |a| a.empty? || a.chars.first == "-" }
       end
 
+      def self.help
+        "audit all modified Casks in a given commit range"
+      end
+
       def self.usage
         <<-EOS.undent
           Usage: brew cask _audit_modified_casks [options...] <commit range>

Library/Homebrew/test/cask/cli_spec.rb

@@ -63,4 +63,10 @@ describe Hbc::CLI, :cask do
       described_class.process("noop")
     end
   end
+
+  it "provides a help message for all commands" do
+    described_class.command_classes.each do |command_class|
+      expect(command_class.help).to match(/\w+/), command_class.name
+    end
+  end
 end