Add tests for callback_shortcuts.0.dart API example. (#147536)

This PR contributes to flutter/flutter#130459

### Description
- Corrects the name for `examples/api/test/widgets/shortcuts/callback_shortcuts.0_test.dart` test file
- Update tests for `examples/api/lib/widgets/shortcuts/callback_shortcuts.0.dart`

Author: ksokolovskyi

dev/bots/check_code_samples.dart

@@ -431,7 +431,6 @@ final Set<String> _knownMissingTests = <String>{
   '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/shortcuts/logical_key_set.0_test.dart',
-  'examples/api/test/widgets/shortcuts/callback_shortcuts.0_test.dart',
   'examples/api/test/widgets/page_storage/page_storage.0_test.dart',
   'examples/api/test/widgets/scrollbar/raw_scrollbar.1_test.dart',
   'examples/api/test/widgets/scrollbar/raw_scrollbar.2_test.dart',

examples/api/test/widgets/shortcuts/callback_shortcuts.0.dart

@@ -0,0 +1,55 @@
+// 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/services.dart';
+import 'package:flutter_api_samples/widgets/shortcuts/callback_shortcuts.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Verify correct labels are displayed', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.CallbackShortcutsApp(),
+    );
+
+    expect(find.text('CallbackShortcuts Sample'), findsOneWidget);
+    expect(
+      find.text('Press the up arrow key to add to the counter'),
+      findsOneWidget,
+    );
+    expect(
+      find.text('Press the down arrow key to subtract from the counter'),
+      findsOneWidget,
+    );
+    expect(find.text('count: 0'), findsOneWidget);
+  });
+
+  testWidgets('Up and down arrow press updates counter', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.CallbackShortcutsApp(),
+    );
+
+    int counter = 0;
+
+    while (counter < 10) {
+      expect(find.text('count: $counter'), findsOneWidget);
+
+      // Increment the counter.
+      await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
+      await tester.pump();
+
+      counter++;
+    }
+
+    while (counter >= 0) {
+      expect(find.text('count: $counter'), findsOneWidget);
+
+      // Decrement the counter.
+      await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
+      await tester.pump();
+
+      counter--;
+    }
+  });
+}