Validate correct string data in fuzz test

src/test.cpp

@@ -170,7 +170,7 @@ void testStreaming(std::string const &json) {
 } // namespace
 
 TEST_CASE("parser3") {
-  Callbacks c = serializeCallbacks();
+  WeaselJsonCallbacks c = serializeCallbacks();
   SerializeState state;
   {
     auto copy = json;