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);
});

Start with the testWidgets example and its FakeCacheManager setup. Check the widget states represented by the imageBuilder, progressIndicatorBuilder, and errorWidget callbacks, then verify the intended state with tester.pumpAndSettle. Done means the test reliably reaches the expected widget identified by the image key.

索引モデルが issue の本文から書いたものです。