Fetch image from memory cache with key

The test should also check that `sut.fetchImage(key)` returns an image equal to `image` pixel by pixel. It doesn't have to be the same object, but it should have the same content.

This helper method from the original Haneke does that:

https://github.com/hpique/Haneke/blob/master/HanekeTests/UIImage%2BHanekeTestUtils.m#L41