nixos/tests: fix systemd-nspawm under network-online dep fix

Changed files
+1
nixos
+1
nixos/tests/systemd-nspawn.nix
···
start_all()
server.wait_for_unit("nginx.service")
+
client.systemctl("start network-online.target")
client.wait_for_unit("network-online.target")
client.succeed("machinectl pull-raw --verify=signature http://server/testimage.raw")
client.succeed(