make sure async test is dispatched

Better than `testing.expectEqual(true, true)`.

src/browser/tests/element/html/event_listeners.html

@@ -493,18 +493,22 @@
 
 <script id="document-element-load">
 {
+  let asyncBlockDispatched = false;
   const docElement = document.documentElement;
 
   testing.async(async () => {
     const result = await new Promise(resolve => {
     // We should get this fired at capturing phase when a resource loaded.
-      docElement.addEventListener("load", (e) => {
+      docElement.addEventListener("load", e => {
         testing.expectEqual(e.eventPhase, Event.CAPTURING_PHASE);
-        resolve(true);
+        return resolve(true);
       }, true);
     });
 
+    asyncBlockDispatched = true;
     testing.expectEqual(true, result);
   });
+
+  testing.eventually(() => testing.expectEqual(true, asyncBlockDispatched));
 }
 </script>