ImageBuilder is never called during the tests in flutter with CachedNetworkImage

Hello,

I'm having an issue with the widget tests using the CachedNetworkImage. The problem is the imageBuilder is never called.You can reproduce the behavior with this test:

```dart
testWidgets("CircleAvatar shown", (WidgetTester tester) async {
await mockNetworkImagesFor(() async {
await tester.pumpWidget(
CachedNetworkImage(
imageUrl: "https://imgur.com/aZyQZTZ",
placeholder: (context, url) => CircularProgressIndicator(),
errorWidget: (context, url, error) => CircleAvatar(child: Icon(Icons.broken_image)),
imageBuilder: (context, imageProvider) => CircleAvatar(backgroundImage: imageProvider),
),
);
expect(find.byType(CachedNetworkImage), findsOneWidget); // OK
expect(find.byType(CircleAvatar), findsOneWidget); // Fail
});
});
```