[rfw] Update tests to no longer depend on a Container pumped via testWidgets. (#9063)

This PR updates `rfw` package tests to no longer depend on a `Container` pumped via `testWidgets` to unblock further work on flutter/flutter#163419

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: ksokolovskyi

packages/rfw/test/runtime_test.dart

@@ -904,7 +904,7 @@ void main() {
     '''));
     expect(
       (runtime.build(
-        tester.element(find.byType(Container)),
+        tester.element(find.byType(View)),
         const FullyQualifiedWidgetName(LibraryName(<String>['test']), 'stateless'),
         data,
         (String eventName, DynamicMap eventArguments) {},
@@ -913,7 +913,7 @@ void main() {
     );
     expect(
       (runtime.build(
-        tester.element(find.byType(Container)),
+        tester.element(find.byType(View)),
         const FullyQualifiedWidgetName(LibraryName(<String>['test']), 'stateful'),
         data,
         (String eventName, DynamicMap eventArguments) {},
@@ -922,7 +922,7 @@ void main() {
     );
     expect(
       (runtime.build(
-        tester.element(find.byType(Container)),
+        tester.element(find.byType(View)),
         const FullyQualifiedWidgetName(LibraryName(<String>['test']), 'switchy'),
         data,
         (String eventName, DynamicMap eventArguments) {},