Get stack traces for errors in most shadowrealm tests

This uses the non-standard `stack` property of Error, if it is present, to
provide stack trace information to the caller of shadowRealmEvalAsync().

The stack trace isn't available in shadowrealm-in-serviceworker and
shadowrealm-in-audioworklet contexts, because they implement a fake
dynamic import with ShadowRealm.prototype.evaluate(), which will wrap any
errors from the imported module in a TypeError and clobber any stack info.
We could do it by adding a try/catch around the module text, but that
would mess with line and column numbers; for now, we'll leave it.

resources/testharness-shadowrealm-outer.js

@@ -5,6 +5,9 @@
  * Convenience function for evaluating some async code in the ShadowRealm and
  * waiting for the result.
  *
+ * In case of error, this function intentionally exposes the stack trace (if it
+ * is available) to the hosting realm, for debugging purposes.
+ *
  * @param {ShadowRealm} realm - the ShadowRealm to evaluate the code in
  * @param {string} asyncBody - the code to evaluate; will be put in the body of
  *   an async function, and must return a value explicitly if a value is to be
@@ -15,7 +18,7 @@ globalThis.shadowRealmEvalAsync = function (realm, asyncBody) {
     (resolve, reject) => {
       (async () => {
         ${asyncBody}
-      })().then(resolve, (e) => reject(e.toString()));
+      })().then(resolve, (e) => reject(e.toString() + "\\n" + (e.stack || "")));
     }
   `));
 };