Support null vs noop comparison.

src/wasm_test/compare.rs

@@ -156,6 +156,9 @@ fn compare_optional_json_value<'b, 's>(
         (None, None) | (None, Some(serde_json::Value::Null)) | (Some(Token::Atom("nil")), None) => {
             Ok(WasmDiffResult::default())
         }
+        (None, Some(serde_json::Value::Object(w))) if w.contains_key("noop") => {
+            Ok(WasmDiffResult::default())
+        }
         (Some(e), Some(w)) => compare_json_value(source, e, w),
         _ => Ok(WasmDiffResult {
             status: vec![WasmDiffStatus::Bad(