fix `<module:MiniTest>': Unit is not a module (TypeError)

http://u64b.rubyci.org/~chkbuild/ruby-trunk/log/20130819T183302Z.log.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42633 b2dd03c8-39d4-4d8f-98ff-823fe69b080e