Make nscd startup synchronous

Nscd forks into the background before it's ready to accept
connections. So explicitly wait until it's ready.

http://hydra.nixos.org/build/10661767

nixos/tests/misc.nix

@@ -13,6 +13,8 @@ import ./make-test.nix {
 
   testScript =
     ''
+      $machine->waitForUnit("default.target");
+
       subtest "nixos-version", sub {
           $machine->succeed("[ `nixos-version | wc -w` = 2 ]");
       };
@@ -61,8 +63,6 @@ import ./make-test.nix {
 
       # Test whether hostname (and by extension nss_myhostname) works.
       subtest "hostname", sub {
-          $machine->waitForUnit("dhcpcd.service");
-          $machine->waitForUnit("network.target");
           $machine->succeed('[ "`hostname`" = machine ]');
           $machine->succeed('[ "`hostname -s`" = machine ]');
       };