Add test for preferred_size.0.dart API example. (flutter#146725)

This PR contributes to flutter#130459

### Description
- Adds `examples/api/test/widgets/preferred_size/preferred_size.0_test.dart` test

dev/bots/check_code_samples.dart

@@ -479,7 +479,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/implicit_animations/sliver_animated_opacity.0_test.dart',
   'examples/api/test/widgets/dismissible/dismissible.0_test.dart',
   'examples/api/test/widgets/scroll_view/custom_scroll_view.1_test.dart',
-  'examples/api/test/widgets/preferred_size/preferred_size.0_test.dart',
   'examples/api/test/widgets/inherited_notifier/inherited_notifier.0_test.dart',
   'examples/api/test/animation/curves/curve2_d.0_test.dart',
   'examples/api/test/gestures/pointer_signal_resolver/pointer_signal_resolver.0_test.dart',

examples/api/test/widgets/preferred_size/preferred_size.0_test.dart

@@ -0,0 +1,35 @@
+// 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/preferred_size/preferred_size.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets(
+    'PreferredSize determines the height of AppBarContent',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.PreferredSizeExampleApp(),
+      );
+
+      final PreferredSize preferredSize = tester.widget(
+        find.ancestor(
+          of: find.byType(example.AppBarContent),
+          matching: find.byType(PreferredSize),
+        ),
+      );
+
+      final RenderBox appBarContent = tester.renderObject(
+        find.byType(example.AppBarContent),
+      ) as RenderBox;
+
+      expect(
+        preferredSize.preferredSize.height,
+        equals(appBarContent.size.height),
+      );
+    },
+  );
+}