Add test for raw_scrollbar.1.dart (flutter#158069)

Contributes to flutter#130459

It adds a test for
- `examples/api/lib/widgets/scrollbar/raw_scrollbar.1.dart`

Author: ValentinVignal

dev/bots/check_code_samples.dart

@@ -312,7 +312,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/color_scheme/dynamic_content_color.0_test.dart',
   '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/scrollbar/raw_scrollbar.1_test.dart',
   'examples/api/test/widgets/scrollbar/raw_scrollbar.2_test.dart',
   'examples/api/test/widgets/scrollbar/raw_scrollbar.desktop.0_test.dart',
   'examples/api/test/widgets/scrollbar/raw_scrollbar.shape.0_test.dart',

examples/api/test/widgets/scrollbar/raw_scrollbar.1_test.dart

@@ -0,0 +1,36 @@
+// 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/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/scrollbar/raw_scrollbar.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('When thumbVisibility is true, the scrollbar thumb remains visible', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.RawScrollbarExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'RawScrollbar Sample'), findsOne);
+    expect(find.byType(RawScrollbar), findsOne);
+    expect(find.byType(GridView), findsOne);
+    expect(find.byType(RawScrollbar), paints..clipRect());
+
+    expect(find.text('item 0'), findsOne);
+    expect(find.text('item 1'), findsOne);
+    expect(find.text('item 2'), findsOne);
+
+    final TestPointer pointer = TestPointer(1, PointerDeviceKind.mouse);
+    pointer.hover(tester.getCenter(find.byType(GridView)));
+    await tester.sendEventToBinding(pointer.scroll(const Offset(0.0, 1000)));
+    await tester.pumpAndSettle();
+
+    expect(find.byType(RawScrollbar), paints..clipRect());
+
+    expect(find.text('item 15'), findsOne);
+    expect(find.text('item 16'), findsOne);
+    expect(find.text('item 17'), findsOne);
+  });
+}