Add test for raw_scrollbar.shape.0.dart (#158094)

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

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

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

@@ -0,0 +1,34 @@
+// 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/scrollbar/raw_scrollbar.shape.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The thumb shape is a stadium border', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ShapeExampleApp(),
+    );
+
+    expect(find.byType(RawScrollbar), findsOne);
+    expect(find.byType(ListView), findsOne);
+
+    expect(find.text('0'), findsOne);
+    expect(find.text('1'), findsOne);
+    expect(find.text('4'), findsOne);
+
+    await tester.pumpAndSettle();
+
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..path(color: Colors.blue)
+        ..rrect(
+          rrect: RRect.fromLTRBR(786.5, 1.5, 798.5, 178.5, const Radius.circular(6)),
+          color: Colors.brown,
+        ),
+    );
+  });
+}