Would be awesome to have a Widget Testing recipe in the readme!

## 💬 Questions and Help

I was trying to follow on old [medium article](https://medium.com/flutter-community/golden-testing-using-cachednetworkimage-1b488c653af3) linked in a [stack overflow question](https://stackoverflow.com/questions/59408845/how-to-write-widget-test-for-cachednetworkimage-in-flutter) to figure out how to write a widget test for a component that relies on CacheManager.

Unfortunately, it seems like a new version significantly changed the API and the recipe provided there is all kinds of broken. I wonder if it would be possible for someone to link from the readme a recipe for how to write such a test (possibly by implementing https://github.com/Baseflow/flutter_cache_manager/blob/develop/flutter_cache_manager/example/test/widget_test.dart and linking to it?)

Start with flutter_cache_manager/example/test/widget_test.dart and compare it with the outdated recipe described in the issue. Check the README to identify where a widget-testing recipe should be linked; done means the example works with the current API and the README points readers to it.

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