testAdd tests for stepper.controls_builder.0.dart (flutter#150669)

Contributes to flutter#130459

It adds a test for
- `examples/api/lib/material/stepper/stepper.controls_builder.0.dart`

Author: ValentinVignal

dev/bots/check_code_samples.dart

@@ -320,7 +320,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/color_scheme/dynamic_content_color.0_test.dart',
   'examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart',
   'examples/api/test/material/menu_anchor/menu_anchor.2_test.dart',
-  'examples/api/test/material/stepper/stepper.controls_builder.0_test.dart',
   'examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart',
   'examples/api/test/material/chip/deletable_chip_attributes.on_deleted.0_test.dart',
   'examples/api/test/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0_test.dart',

examples/api/test/material/stepper/stepper.controls_builder.0_test.dart

@@ -0,0 +1,21 @@
+// 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/material/stepper/stepper.controls_builder.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Stepper control builder can be overridden to display custom buttons', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ControlsBuilderExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'Stepper Sample'), findsOne);
+    expect(find.text('A').hitTestable(), findsOne);
+    expect(find.text('B').hitTestable(), findsOne);
+    expect(find.widgetWithText(TextButton, 'NEXT').hitTestable(), findsOne);
+    expect(find.widgetWithText(TextButton, 'CANCEL').hitTestable(), findsOne);
+  });
+}