test(): Update the dummy classifications we use for testing and test artifact content (#53)

The goal of this MR, which should close issue #53 (closed) , is to test the plugin with a minimal change example. The minimal change example has two 3x3 LULC rasters that have all change categories (no change, change, unknown). Also, it is tested that the content of the artifacts matches the expectations. Issue #53 (closed) was spawned from this discussion.

@moritz please also take a look at it, thanks :)