Add tests for ordered_traversal_group.0.dart (#152849)

Contributes to https://github.com/flutter/flutter/issues/130459

It adds a test for
- `examples/api/lib/widgets/focus_traversal/focus_traversal_group.0.dart`

dev/bots/check_code_samples.dart

@@ -343,7 +343,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/scroll_position/scroll_metrics_notification.0_test.dart',
   'examples/api/test/widgets/media_query/media_query_data.system_gesture_insets.0_test.dart',
   'examples/api/test/widgets/animated_list/animated_list.0_test.dart',
-  'examples/api/test/widgets/focus_traversal/ordered_traversal_policy.0_test.dart',
   'examples/api/test/widgets/image/image.frame_builder.0_test.dart',
   'examples/api/test/widgets/image/image.loading_builder.0_test.dart',
   'examples/api/test/widgets/page_storage/page_storage.0_test.dart',

examples/api/test/widgets/focus_traversal/focus_traversal_group.0_test.dart

@@ -8,9 +8,9 @@ import 'package:flutter_api_samples/widgets/focus_traversal/focus_traversal_grou
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-    bool hasFocus(WidgetTester tester, String text) {
+  bool hasFocus(WidgetTester tester, String text) {
-      return Focus.of(tester.element(find.text(text))).hasPrimaryFocus;
+    return Focus.of(tester.element(find.text(text))).hasPrimaryFocus;
-    }
+  }
 
   testWidgets('The focus updates should follow the focus traversal groups policy', (WidgetTester tester) async {
     await tester.pumpWidget(

examples/api/test/widgets/focus_traversal/ordered_traversal_policy.0_test.dart

@@ -0,0 +1,38 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_api_samples/widgets/focus_traversal/ordered_traversal_policy.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  bool hasFocus(WidgetTester tester, String text) {
+    return Focus.of(tester.element(find.text(text))).hasPrimaryFocus;
+  }
+
+  testWidgets('The focus updates should follow the focus traversal groups policy', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OrderedTraversalPolicyExampleApp(),
+    );
+
+    expect(hasFocus(tester, 'One'), isTrue);
+
+    const List<String> focusOrder = <String>[
+      'Two',
+      'Three',
+      'Four',
+      'Five',
+      'Six',
+      'One',
+    ];
+
+    for (final String text in focusOrder) {
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.pump();
+
+      expect(hasFocus(tester, text), isTrue);
+    }
+  });
+}