[Testing] allow setup to have access to WidgetTester, via setUpWidgets

In tests, some setup could be set up if the `setUp` function gave access to the widget tester, or alternatively if a `setUpWidgets` was added.

```dart

setUpWidgets((tester) async {
   await tester.pumpWidget(SomeWidget());
});

```

I've done a quick package to do this: https://github.com/cedvdb/flutter_test_extended in the maintime but the implementation is just plain bad