Use ::Syck explicitly.

lib/rubygems/requirement.rb defines YAML::Syck.
So explicitly specify the top level one.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32699 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/psych/test_engine_manager.rb

@@ -21,7 +21,7 @@ module Psych
 
    def test_set_syck
      YAML::ENGINE.yamler = 'syck'
-     assert_equal Syck, YAML
+     assert_equal ::Syck, YAML
      assert_equal 'syck', YAML::ENGINE.yamler
    end