test: add mockito

frontend/app_flowy/packages/appflowy_editor/pubspec.yaml

@@ -32,6 +32,7 @@ dev_dependencies:
     sdk: flutter
   flutter_lints: ^2.0.1
   network_image_mock: ^2.1.1
+  mockito: ^5.3.2
 
 # For information on the generic Dart part of this file, see the
 # following page: https://dart.dev/tools/pub/pubspec