Use a null default skip value in testWidgets so it can inherit the group's skip flag (#76174)

packages/flutter_test/lib/src/widget_tester.dart

@@ -108,7 +108,7 @@ typedef WidgetTesterCallback = Future<void> Function(WidgetTester widgetTester);
 void testWidgets(
   String description,
   WidgetTesterCallback callback, {
-  bool skip = false,
+  bool? skip,
   test_package.Timeout? timeout,
   Duration? initialTimeout,
   bool semanticsEnabled = true,

packages/flutter_test/test/widget_tester_test.dart

@@ -52,6 +52,12 @@ void main() {
     });
   });
 
+  group('respects the group skip flag', () {
+    testWidgets('should be skipped', (WidgetTester tester) async {
+      expect(false, true);
+    });
+  }, skip: true);
+
   group('findsOneWidget', () {
     testWidgets('finds exactly one widget', (WidgetTester tester) async {
       await tester.pumpWidget(const Text('foo', textDirection: TextDirection.ltr));