test/module.rb: call #prepend_features via #__send__ to skip private check

test/t/module.rb

@@ -473,9 +473,9 @@ assert 'Module#prepend_features' do
   mod = Module.new { def m; :mod end }
   cls = Class.new { def m; :cls end }
   assert_equal :cls, cls.new.m
-  mod.prepend_features(cls)
+  mod.__send__(:prepend_features,cls)
   assert_equal :mod, cls.new.m
-  assert_raise(FrozenError) { Module.new.prepend_features(Class.new.freeze) }
+  assert_raise(FrozenError) { Module.new.__send__(:prepend_features,Class.new.freeze) }
 end
 
 # @!group prepend