test: fix WPT.test()'s error handling

Changing err.message after the construction of Error doesn't seem to
change err.stack.

PR-URL: https://github.com/nodejs/node/pull/11436
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common.js

@@ -604,8 +604,7 @@ exports.WPT = {
     try {
       fn();
     } catch (err) {
-      if (err instanceof Error)
+      console.error(`In ${desc}:`);
-        err.message = `In ${desc}:\n  ${err.message}`;
       throw err;
     }
   },