fix(text): Fix UI test failures

A new UI test added in a recent CL involved manually setting the
currentTime value of a videoElement in tests.
For some reason that wasn't working on Safari, so this changes that
UI test to use a fake video instead of a real one.

Closes shaka-project#3689

Change-Id: I412f677f6cda49bbfc850a9102edfe3b75bca302

test/text/ui_text_displayer_unit.js

@@ -4,7 +4,6 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-goog.require('shaka.test.UiUtils');
 goog.require('shaka.test.Util');
 goog.require('shaka.text.Cue');
 goog.require('shaka.text.UITextDisplayer');
@@ -51,8 +50,7 @@ describe('UITextDisplayer', () => {
       /** @type {!HTMLElement} */ (document.createElement('div'));
     videoContainer.style.height = `${videoContainerHeight}px`;
     document.body.appendChild(videoContainer);
-    video = shaka.test.UiUtils.createVideoElement();
-    videoContainer.appendChild(video);
+    video = new shaka.test.FakeVideo();
   });
 
   beforeEach(() => {