nixos/release-combined: Add keymap tests

We really want to break channel updates whenever we break something like
this, because this actually will hit machines out there and can be very
much annoying (we had broken keymaps a few times which is why I
introduced these tests in the first place).

Just to be sure I don't break channel updates with this commit, I ran
all of the keymap tests and they all succeeded.

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