Add test for focus_traversal_group.0.dart (#151591)

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`

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

@@ -0,0 +1,45 @@
+// 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/focus_traversal_group.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.FocusTraversalGroupExampleApp(),
+    );
+
+    // Set the focus to the first button.
+    Focus.of(tester.element(find.text('num: 0'))).requestFocus();
+    await tester.pump();
+
+    expect(hasFocus(tester, 'num: 0'), isTrue);
+
+    const List<String> focusOrder = <String>[
+      'num: 1',
+      'num: 2',
+      'String: A',
+      'String: B',
+      'String: C',
+      'ignored num: 3',
+      'ignored num: 2',
+      'ignored num: 1',
+      'num: 0',
+    ];
+
+    for (final String text in focusOrder) {
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.pump();
+
+      expect(hasFocus(tester, text), isTrue);
+    }
+  });
+}