nixos/mosquitto: use networkd for tests

the non-networkd backend does not wait for slaac to finish (ie, ipv6
addresses coming out of tentative state), and that breaks the mosquitto
bind_interface test slightly. if slaac takes too long the test will run
into mosquitto restart limits and fail.

pennae 5f5b5a76 e8ce641a

Changed files
+1
nixos
+1
nixos/tests/mosquitto.nix
···
in {
server = { pkgs, ... }: {
networking.firewall.allowedTCPPorts = [ port tlsPort anonPort ];
+
networking.useNetworkd = true;
services.mosquitto = {
enable = true;
settings = {