I avoid using the same module for library in test because
it provides "include REXML" environment in test. Normally,
users don't use REXML on "include REXML" environment. So I
don't want to write tests on "include REXML" environment.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46171 b2dd03c8-39d4-4d8f-98ff-823fe69b080e