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

+1
+1
nixos/tests/nfs/kerberos.nix
··· 105 105 server.wait_for_unit("rpc-gssd.service") 106 106 server.wait_for_unit("rpc-svcgssd.service") 107 107 108 + client.systemctl("start network-online.target") 108 109 client.wait_for_unit("network-online.target") 109 110 110 111 # add principals to client keytab