Updated rfw/test/material_widgets_test.dart for M3 (#4316)

The appearance of the golden image created by the  rfw `'Material widgets'` test depends on the Material2 theme, which is currently the default theme. Updated the test to ensure that the test continues to succeed when the default theme is changed to Material3.

See flutter/flutter#127064

Author: HansMuller

packages/rfw/test/material_widgets_test.dart

@@ -24,6 +24,7 @@ void main() {
     final List<String> eventLog = <String>[];
     await tester.pumpWidget(
       MaterialApp(
+        theme: ThemeData(useMaterial3: false),
         home: RemoteWidget(
           runtime: runtime,
           data: data,