tests: add mocha mocking library

Library/Homebrew/test/testing_env.rb

@@ -65,13 +65,18 @@ unless ARGV.include? "--no-compat" or ENV['HOMEBREW_NO_COMPAT']
 end
 
 require 'test/unit' # must be after at_exit
-
 require 'extend/ARGV' # needs to be after test/unit to avoid conflict with OptionsParser
-ARGV.extend(HomebrewArgvExtension)
-
 require 'extend/ENV'
+ARGV.extend(HomebrewArgvExtension)
 ENV.extend(HomebrewEnvExtension)
 
+begin
+  require 'rubygems'
+  require 'mocha/setup'
+rescue LoadError
+  warn 'The mocha gem is required to run some tests, expect failures'
+end
+
 module VersionAssertions
   def version v
     Version.new(v)