message: Fix subobject-linkage warning
diff: @@ -240,6 +240,7 @@ struct is_message {
  } // namespace <anonymous>
  #define SERIALIZATION_TEST(name) \
  + namespace { \
  template <class Serializer, class Deserializer> \
  struct name##_tpl : fixture<Serializer, Deserializer> { \
  using super = fixture<Serializer, Deserializer>; \
  @@ -263,7 +264,6 @@ struct is_message {
  using super::msg_roundtrip; \
  void run_test_impl(); \
  }; \
  - namespace { \
  using name##_binary = name##_tpl<binary_serializer, binary_deserializer>; \
  using name##_stream = name##_tpl<stream_serializer<vectorbuf>, \
  stream_deserializer<charbuf>>; \
  