message: test: Fix message tests to only run when using serialization
diff: @@ -719,20 +719,20 @@ mod test {
  const ERROR_STR: &str =
  "{\"Error\":{\"Id\":0,\"ErrorCode\":1,\"ErrorMessage\":\"Test Error\"}}";
  - #[test]
  + #[cfg_attr(feature = "serialize_json", test)]
  fn test_ok_serialize() {
  let ok = ButtplugMessageUnion::Ok(Ok::new(0));
  let js = serde_json::to_string(&ok).unwrap();
  assert_eq!(OK_STR, js);
  }
  - #[test]
  + #[cfg_attr(feature = "serialize_json", test)]
  fn test_ok_deserialize() {
  let union: ButtplugMessageUnion = serde_json::from_str(&OK_STR).unwrap();
  assert_eq!(ButtplugMessageUnion::Ok(Ok::new(0)), union);
  }
  - #[test]
  + #[cfg_attr(feature = "serialize_json", test)]
  fn test_error_serialize() {
  let error =
  ButtplugMessageUnion::Error(Error::new(ErrorCode::ErrorHandshake, "Test Error"));
  @@ -740,7 +740,7 @@ mod test {
  assert_eq!(ERROR_STR, js);
  }
  - #[test]
  + #[cfg_attr(feature = "serialize_json", test)]
  fn test_error_deserialize() {
  let union: ButtplugMessageUnion = serde_json::from_str(&ERROR_STR).unwrap();
  assert_eq!(
  