Bug 1854726 [wpt PR 42105] - Fix all-with-discrete transition test, a=testonly

Automatic update from web-platform-tests
Fix all-with-discrete transition test

Issues with the test:
* TransitionStart was being tested before confirming that the
  animation was ready. The timing of when the animation is ready is
  UA dependent.
* Waiting a single frame after starting the animation ensures the
  transtionstart event is queued, but event dispatch could be in the
  next frame

We can avoid the timing nuances by simply querying all animations.
The query itself forces a style update. The animation will be in
a play-pending state at this point so the transitionstart has not
had a chance to fire; however, we can verify that the animation is
a CSSTransition for the expected property.

This test still fails on Firefox, but that is due to not supporting
"allow-discrete". The test passes reliably in Chrome.

Bug: 1477833
Change-Id: I4360d7f66e25b5026a17d332a8f6a3af7482d2dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4880561
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1199763}

--

wpt-commits: 6e5d0de739160bf99505eea5381a1981f913a323
wpt-pr: 42105

testing/web-platform/tests/css/css-transitions/all-with-discrete.tentative.html

@@ -22,13 +22,11 @@
 
 <script>
 promise_test(async () => {
-  let transitionstartFired = false;
-  target.addEventListener('transitionstart', () => {
-    transitionstartFired = true;
-  });
   await waitForAnimationFrames(2);
   target.classList.add('animated');
-  await waitForAnimationFrames(1);
-  assert_true(transitionstartFired);
+  const anims = document.getAnimations();
+  assert_equals(anims.length, 1,
+                "Did not start a discrete-property transition");
+  assert_equals(anims[0].transitionProperty, 'float');
 }, 'transition:all with transition-behavior:allow-discrete should animate discrete properties.');
 </script>