Added missing tests for OverflowBar example. (flutter#146780)

Added missing tests for OverflowBar example. Issue flutter#130459

dev/bots/check_code_samples.dart

@@ -463,7 +463,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/tween_animation_builder/tween_animation_builder.0_test.dart',
   'examples/api/test/widgets/single_child_scroll_view/single_child_scroll_view.1_test.dart',
   'examples/api/test/widgets/single_child_scroll_view/single_child_scroll_view.0_test.dart',
-  'examples/api/test/widgets/overflow_bar/overflow_bar.0_test.dart',
   'examples/api/test/widgets/restoration/restoration_mixin.0_test.dart',
   'examples/api/test/widgets/actions/actions.0_test.dart',
   'examples/api/test/widgets/actions/action_listener.0_test.dart',

examples/api/test/widgets/overflow_bar/overflow_bar.0_test.dart

@@ -0,0 +1,28 @@
+// 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_api_samples/widgets/overflow_bar/overflow_bar.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('OverflowBar displays buttons', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OverflowBarExampleApp(),
+    );
+
+    // Creates a finder that matches widgets of the given
+    // `widgetType`, ensuring that the given widgets exist
+    // inside of an OverflowBar.
+    Finder buttonsFinder(Type widgetType) {
+      return find.descendant(
+        of: find.byType(OverflowBar),
+        matching: find.byType(widgetType),
+      );
+    }
+
+    expect(buttonsFinder(TextButton), findsNWidgets(2));
+    expect(buttonsFinder(OutlinedButton), findsOne);
+  });
+}