How to test imagebuilder

## 💬 Questions and Help

I need to test imageBuilder, I have this code, but I can't.

```dart
testWidgets('Cached networked image builder', (tester) async {
final widget = CachedNetworkImage(
cacheManager: cacheManager, //<<< FakeCacheManager
imageBuilder: (
context,
url,
) =>
Container(
key: Key('image'),
),
progressIndicatorBuilder: (context, url, progress) => Container(
key: Key('progress'),
),
errorWidget: (context, url, error) => Container(
key: Key('failed'),
),
imageUrl: 'foo.nl',
);
await tester.pumpWidget(
MediaQuery(
data: MediaQueryData(),
child: MaterialApp(
home: widget,
),
),
);

await tester.pumpAndSettle();

expect(find.byKey(Key('image')), findsOneWidget);
});

从 testWidgets 示例及其 FakeCacheManager 设置开始。检查由 imageBuilder、progressIndicatorBuilder 和 errorWidget 回调表示的 widget 状态，然后使用 tester.pumpAndSettle 验证预期状态。完成的标准是测试能够可靠地到达由 image key 标识的预期 widget。

由索引模型根据 Issue 内容生成。