LibWeb: Make request-animation-frame-order test async

Even though this test worked fine, it does use requestAnimationFrame
callbacks, so lets make it async to ensure it doesn't timeout.

Tests/LibWeb/Text/input/request-animation-frame-order.html

@@ -2,12 +2,20 @@
 <div id="foo"></div>
 <script src="include.js"></script>
 <script>
-    test(() => {
+    asyncTest(done => {
+        let animFrameCallCount = 0;
+        function animFrameCb(i) {
+            println(i);
+            animFrameCallCount++;
+            if (animFrameCallCount === 20)
+                done();
+        }
+
         for (let i = 0; i < 20; i++) {
             // FIXME: Workaround for https://github.com/SerenityOS/serenity/issues/23552
             let x = i;
             requestAnimationFrame(() => {
-                println(x);
+                animFrameCb(x);
             });
         }
     });