nixos: Add small VM test for lightdm.

Serves as a regression test for #7902.

It's not yet referenced in release(-combined)?.nix because it will fail
until the issue is resolved. Tested successfully against libgcrypt with
libcap passed as null however.

As for the test itself, I'm not quite sure whether checking for the time
displayed by IceWM is a good idea, but we can still fix that if it turns
out to be a problem.

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