Hide the debug banner in the PopupMenuButton example (#108324)

* Hide the debug banner in the PopupMenuButton example

* Add test to verify that debug banner is not shown

examples/api/lib/material/popupmenu/popupmenu.0.dart

@@ -19,6 +19,7 @@ class MyApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return const MaterialApp(
+      debugShowCheckedModeBanner: false,
       title: _title,
       home: MyStatefulWidget(),
     );

examples/api/test/material/popupmenu/popupmenu.0.test.dart

@@ -26,4 +26,9 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('_selectedMenu: itemOne'), findsNothing);
   });
+
+  testWidgets('Does not show debug banner', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.MyApp());
+    expect(find.byType(CheckedModeBanner), findsNothing);
+  });
 }