Remove a warning to load mocha gem in test

For this message.

    *** Mocha deprecation warning: Change `require mocha` to `require mocha/setup`.

Author: kachick

test/common.rb

@@ -1,5 +1,5 @@
 require 'test/unit'
-require 'mocha'
+require 'mocha/setup'
 require 'stringio'
 
 begin