Add test for scroll_metrics_notification.0.dart (#157768)

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

It adds a test for
- `examples/api/lib/widgets/scroll_position/scroll_metrics_notification.0.dart`

examples/api/test/widgets/scroll_position/scroll_metrics_notification.0_test.dart

@@ -0,0 +1,24 @@
+// 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/scroll_position/scroll_metrics_notification.0.dart'
+as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('A snackbar is shown when the scroll metrics change', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ScrollMetricsDemo(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'ScrollMetrics Demo'), findsOne);
+    expect(find.byType(FlutterLogo), findsOne);
+
+    await tester.tap(find.widgetWithIcon(FloatingActionButton, Icons.add));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(SnackBar, 'Scroll metrics changed!'), findsOne);
+  });
+}