Add test to reproduce the issue reported in GH-66

Author: mrkn

test/fiddle/test_cstruct.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+begin
+  require_relative 'helper'
+rescue LoadError
+end
+
+module Fiddle
+  class TestStruct < TestCase
+    # https://github.com/ruby/fiddle/issues/66
+    def test_clone_gh_66
+      s = Fiddle::Importer.struct(["int i"])
+      a = s.malloc
+      a.i = 10
+      b = a.clone
+      b.i = 20
+      assert_equal({a: 10,  b: 20},
+                   {a: a.i, b: b.i})
+    end
+  end
+end