Add tests for focusable_action_detector.0.dart (#157575)

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

It adds a test for
- `examples/api/lib/widgets/actions/focusable_action_detector.0.dart`

examples/api/test/widgets/actions/focusable_action_detector.0_test.dart

@@ -0,0 +1,69 @@
+// 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/actions/focusable_action_detector.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  final Finder redContainerFinder = find.byWidgetPredicate(
+    (Widget widget) => widget is Container && widget.color == Colors.red,
+  );
+
+  testWidgets('Taps on the "And me" button toggle the red box', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FocusableActionDetectorExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'FocusableActionDetector Example'), findsOne);
+
+    expect(redContainerFinder, findsNothing);
+
+    await tester.tap(find.text('And Me'));
+    await tester.pump();
+
+    expect(redContainerFinder, findsOne);
+
+    await tester.tap(find.text('And Me'));
+    await tester.pump();
+
+    expect(redContainerFinder, findsNothing);
+  });
+
+  testWidgets('Hits on the X key when "And me" is focused toggle the red box', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FocusableActionDetectorExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'FocusableActionDetector Example'), findsOne);
+
+    expect(redContainerFinder, findsNothing);
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.tab); // Focuses the "Press Me" button.
+    await tester.pump();
+
+    expect(redContainerFinder, findsNothing);
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyX);
+    await tester.pump();
+
+    expect(redContainerFinder, findsNothing);
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.tab); // Focuses the "And Me" button.
+    await tester.pump();
+
+    expect(redContainerFinder, findsNothing);
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyX);
+    await tester.pump();
+
+    expect(redContainerFinder, findsOne);
+
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyX);
+    await tester.pump();
+
+    expect(redContainerFinder, findsNothing);
+  });
+}