Enable local screenshot testing

[Korijn]

To enable local screenshot testing, we can make the similarity test more forgiving for small errors.

Standard practise is to use the NRMSE metric (skimage.metrics.normalized_root_mse) and set a threshold (e.g. 0.2).

This won't enable local screenshot generation, but at least you can make the test a little more comprehensive than just checking if the examples run or not.