Tests/LibWeb: Fix iframe-load-event-for-bad-xml.html test

This was supposed to be an async test (and work correctly, too!)

This fixes up fa433219388280e0a3b24759f3cb0ed7639ff4a1

Tests/LibWeb/Text/expected/HTML/iframe-load-event-for-bad-xml.txt

@@ -1 +1 @@
-   
+   OK

Tests/LibWeb/Text/input/HTML/iframe-load-event-for-bad-xml.html

@@ -1,10 +1,11 @@
 <iframe id="i1"></iframe>
 <script src="../include.js"></script>
 <script>
-    test(() => {
+    asyncTest((done) => {
         i1.src = "data:application/xml;charset=utf-8;base64,vwo=";
         i1.onload = function() {
             println("OK");
+            done();
         }
     });
 </script>