test: small serialization test of u256 into json (#991)

Co-authored-by: lambdaclass-user <github@lambdaclass.com>

crates/primitives/src/jsonu256.rs

@@ -90,4 +90,12 @@ mod test {
             ]
         );
     }
+
+    #[test]
+    fn jsonu256_serialize() {
+        let data = JsonU256(U256::from(16));
+        let serialized = serde_json::to_string(&data).unwrap();
+
+        assert_eq!(serialized, r#""16""#);
+    }
 }