[video_player] Fix order-dependent tests (#5672)

Author: stuartmorgan-g

packages/video_player/video_player/CHANGELOG.md

@@ -1,3 +1,7 @@
+## NEXT
+
+* Fixes order-dependent unit tests.
+
 ## 2.4.2
 
 * Minor fixes for new analysis options.

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(''),