✅ [Story player] Fix animation unit test flakiness (ampproject#34382)

* added animation options

* Removed animation from behaviordef

* Added newline on player

* Added documentation on options

* Use next to toggle animation

* Use transitionend

* Using classes for CSS

* Reverted requestAnimationFrame promise

* Added tests

* Apply suggestions from code review

Co-authored-by: Enrique Marroquin <5449100+Enriqe@users.noreply.github.com>

* Use true instead of property

Co-authored-by: Enrique Marroquin <5449100+Enriqe@users.noreply.github.com>

* Removed trailing whitespace

* Fixed linting

* Wait for transitionend

Co-authored-by: Enrique Marroquin <5449100+Enriqe@users.noreply.github.com>

test/unit/test-amp-story-player.js

@@ -19,6 +19,8 @@ import {AmpStoryPlayer} from '../../src/amp-story-player/amp-story-player-impl';
 import {Messaging} from '@ampproject/viewer-messaging';
 import {PageScroller} from '../../src/amp-story-player/page-scroller';
 import {expect} from 'chai';
+import {listenOncePromise} from '../../src/event-helper';
+import {macroTask} from '../../testing/yield';
 
 describes.realWin('AmpStoryPlayer', {amp: false}, (env) => {
  let win;
@@ -1499,7 +1501,8 @@ describes.realWin('AmpStoryPlayer', {amp: false}, (env) => {
  .querySelector('.i-amphtml-story-player-main-container');
 
  // Wait for event dispatched to be listened
- await new Promise((resolve) => setTimeout(() => resolve(), 50));
+ await listenOncePromise(playerEl, 'transitionend');
+ await macroTask();
 
  expect(
  rootEl.classList.contains(