test_module.rb: use assertions for messages

* test/ruby/test_module.rb (TestModule#assert_top_method_is_private):
  use assert_separately and assert_raise_with_message for better
  messages.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43018 b2dd03c8-39d4-4d8f-98ff-823fe69b080e