[video_player] Fix order-dependent tests

Tests in this group use VideoPlayerController, which relies on the
`VideoPlayerPlatform.instance`. That was not being set in this group,
only an earlier group, which caused the tests to be order-dependent.

Fixes b/230943469

packages/video_player/video_player/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## NEXT
 
 * Removes unnecessary imports.
+* Fixes order-dependent unit tests.
 
 ## 2.4.0
 

packages/video_player/video_player/test/video_player_test.dart

@@ -958,6 +958,13 @@ void main() {
   });
 
   group('VideoPlayerOptions', () {
+    late FakeVideoPlayerPlatform fakeVideoPlayerPlatform;
+
+    setUp(() {
+      fakeVideoPlayerPlatform = FakeVideoPlayerPlatform();
+      VideoPlayerPlatform.instance = fakeVideoPlayerPlatform;
+    });
+
     test('setMixWithOthers', () async {
       final VideoPlayerController controller = VideoPlayerController.file(
           File(''),