Prototype porting to xtest_data

[HeroicKatora]

This adds integration of xtest-data. This crate ensures that all of our tests suites can be executed with the released packages on crates.io—even though they do not contain the image data etc. It solves this by smartly fetching them from our repository if required.

This is part of series of PRs on the other image crates to find out how the usability of xtest-data is and how it could be improved.