Fix alias method from included module on 2.1 and 2.0.0-p451

@floehopper
I'd prefer it if we didn't use this MiniTest-specific method. It ought to be possible to use the `#run_as_tests` mechanism as in https://github.com/freerange/mocha/blob/master/test/acceptance/stubbing_same_class_method_on_parent_and_child_classes_test.rb#L23..L32.