add test for public_methods(false)

test/t/kernel.rb

@@ -423,6 +423,11 @@ end
 
 assert('Kernel#public_methods', '15.3.1.3.38') do
   assert_equal Array, public_methods.class
+  class Foo
+    def foo
+    end
+  end
+  assert_equal [:foo], Foo.new.public_methods(false)
 end
 
 # Kernel#puts is defined in mruby-print mrbgem. '15.3.1.3.39'