Remove assigned but unused variable warning

Author: etiennebarrie

test/ruby/test_module.rb

@@ -418,6 +418,7 @@ def test_initialize_copy
     instance = klass.new
     assert_equal(:first, instance.foo)
     new_mod = Module.new { define_method(:foo) { :second } }
+    new_mod = new_mod # keep a reference
     4.times { GC.start }
     assert_equal(:first, instance.foo) # [BUG] unreachable
   end