don't call gem method if Test::Unit is defined

ruby's test-all may define 'gem' method somewhere,
and it cause test failure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43772 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rdoc/test_case.rb

@@ -1,7 +1,7 @@
 require 'rubygems'
 
 begin
-  gem 'minitest', '~> 4.0'
+  gem 'minitest', '~> 4.0' unless defined?(Test::Unit)
 rescue NoMethodError
   # for ruby tests
 end