LibWeb: Use InternalAnimationTimeline in existing tests

Author: https://github.com/mattco98
Commit: 328ad9a2f0
Pull-request: https://github.com/SerenityOS/serenity/pull/23774

Tests/LibWeb/Text/input/WebAnimations/animation-properties/currentTime.html

@@ -2,7 +2,7 @@
 <div id="foo"></div>
 <script src="../../include.js"></script>
 <script>
-    promiseTest(async () => {
+    test(() => {
         const foo = document.getElementById("foo");
 
         let animation = new Animation(null, null);
@@ -11,14 +11,13 @@
         animation = new Animation(new KeyframeEffect(foo, []));
         println(`Animation that hasn't been played has null currentTime: ${animation.currentTime === null}`);
 
-        animation = foo.animate({ color: "red" }, { duration: 1000 });
+        const timeline = internals.createInternalAnimationTimeline();
+        animation = foo.animate({ color: "red" }, { duration: 1000, timeline });
         println(`Played animation has a currentTime of 0: ${animation.currentTime === 0}`);
 
-        await timeout(100);
-
-        // FIXME: Figure out how to consistently test timings
-        // if (animation.currentTime > 95 && animation.currentTime < 105)
-        //     println("Animation time after 100ms is correct");
+        timeline.setTime(100);
+        if (animation.currentTime === 100)
+            println("Animation time after 100ms is correct");
 
         animation = foo.animate({ opacity: [0, 1] }, { duration: 1000 });
         println(`New animation has not started animating: ${getComputedStyle(foo).opacity === "0"}`);