nixos/tests: fix nfs/kerberos under network-online dep fix [BROKEN]

nixos/tests/nfs/kerberos.nix

···
       server.wait_for_unit("rpc-gssd.service")
       server.wait_for_unit("rpc-svcgssd.service")
 
+      client.systemctl("start network-online.target")
       client.wait_for_unit("network-online.target")
 
       # add principals to client keytab