Load save files for tests

[danielbrauer]

Some tests need setup, or rely on a specific set of data being in a save. For example, backwards compatibility tests could be made by loading a save from an earlier version of the mod.

To this end, it would be much better to be able to load a file automatically for a given set of tests than to have to do manual setup each time.

[JonasJurczok]

Yep. That will be possible with #15 :)