achieve over 90% test coverage of dln.c.
* test/ruby/test_class.rb: add tests to achieve over 90% test coverage
of class.c.
* test/ruby/test_module.rb: ditto.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@16510 b2dd03c8-39d4-4d8f-98ff-823fe69b080e