[webview_flutter] Update Widget Unit Tests to be platform agnostic (#4489)

Author: bparrishMines

packages/webview_flutter/webview_flutter/pubspec.yaml

@@ -24,8 +24,10 @@ dependencies:
   webview_flutter_wkwebview: ^2.2.0
 
 dev_dependencies:
+  build_runner: ^2.1.5
   flutter_driver:
     sdk: flutter
   flutter_test:
     sdk: flutter
+  mockito: ^5.0.16
   pedantic: ^1.10.0