Add test for raw_scrollbar.0.dart (#157989)

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

It adds a test for
- `examples/api/lib/widgets/scrollbar/raw_scrollbar.0.dart`

examples/api/test/widgets/scrollbar/raw_scrollbar.0_test.dart

@@ -0,0 +1,37 @@
+// 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.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('There are two scrollbars', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.RawScrollbarExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'RawScrollbar Sample'), findsOne);
+    expect(find.byType(Scrollbar), findsExactly(2));
+
+    expect(find.text('Scrollable 1 : Index 0'), findsOne);
+    expect(find.text('Scrollable 2 : Index 0'), findsOne);
+
+    final TestPointer pointer = TestPointer(1, PointerDeviceKind.mouse);
+    pointer.hover(tester.getCenter(find.byType(ListView).first));
+    await tester.sendEventToBinding(pointer.scroll(const Offset(0.0, 1000)));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Scrollable 1 : Index 40'), findsOne);
+    expect(find.text('Scrollable 2 : Index 0'), findsOne);
+
+    pointer.hover(tester.getCenter(find.byType(ListView).last));
+    await tester.sendEventToBinding(pointer.scroll(const Offset(0.0, 1000)));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Scrollable 1 : Index 40'), findsOne);
+    expect(find.text('Scrollable 2 : Index 20'), findsOne);
+  });
+}