Disable the cjdns test

The cjdns module uses "import from derivation", which is a bad thing
(it causes building at evaluation time).

nixos/release.nix

···
   tests.blivet = callTest tests/blivet.nix {};
   tests.cadvisor = scrubDrv (import tests/cadvisor.nix { system = "x86_64-linux"; });
   tests.chromium = callTest tests/chromium.nix {};
-  tests.cjdns = callTest tests/cjdns.nix {};
+  #tests.cjdns = callTest tests/cjdns.nix {};
   tests.containers = callTest tests/containers.nix {};
   tests.docker = scrubDrv (import tests/docker.nix { system = "x86_64-linux"; });
   tests.dockerRegistry = scrubDrv (import tests/docker-registry.nix { system = "x86_64-linux"; });