Correct postMessage() test

This test was passing but no assert was being run.

Fixes #29536.

webmessaging/with-options/one-arg.html

@@ -4,11 +4,10 @@
 <script src="/resources/testharnessreport.js"></script>
 <div id="log"></div>
 <script>
-test(function() {
+async_test(t => {
   postMessage('');
-  onmessage = this.step_func(function(e) {
+  onmessage = t.step_func_done(e => {
     assert_equals(e.data, '');
-    this.done();
   });
 });
 </script>