merges r28174 from trunk into ruby_1_9_2.

--
add an assertion message.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@28276 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_rand.rb

@@ -398,7 +398,7 @@ END
         raise 'default seed is not set' if srand == 0
     end
     p2, st = Process.waitpid2(pid)
-    assert(st.success?)
+    assert(st.success?, "#{st.inspect}")
   rescue NotImplementedError, ArgumentError
   end