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

Changed files
+1
nixos
tests
+1
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