nixos: Add a VM test for the SLiM display manager

This is currently our default display manager, so I'm adding this to the
"tested" job as well to ensure we don't ship broken revisions where X is
most likely not working.

The test uses a custom SLiM theme that's specifically tailored for good
OCR results (mainly white background and black fonts without anything
else), because our default NixOS theme has a very small contrast between
background and fonts in some places.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>