nixos/tests: Temporarily deactivate nixpart tests.

Those tests are flapping and redundant to some degree, as two
configurations are tested in NixOps as well. So, let's deactivate them
until the 1.0 release of nixpart, which has a more general approach for
automatically partitioning NixOS installations.

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