nixos/sssd: create symlinks in /etc to fix sssctl

Without this, sssctl fails to read its configuration. Update the NixOS
test to ensure sssctl doesn't regress.

nixos/modules/services/misc/sssd.nix

···
   };
   config = mkMerge [
     (mkIf cfg.enable {
+      # For `sssctl` to work.
+      environment.etc."sssd/sssd.conf".source = settingsFile;
+      environment.etc."sssd/conf.d".source = "${dataDir}/conf.d";
+
       systemd.services.sssd = {
         description = "System Security Services Daemon";
         wantedBy    = [ "multi-user.target" ];
···
           EnvironmentFile = lib.mkIf (cfg.environmentFile != null) cfg.environmentFile;
         };
         preStart = ''
+          mkdir -p "${dataDir}/conf.d"
           [ -f ${settingsFile} ] && rm -f ${settingsFile}
           old_umask=$(umask)
           umask 0177

nixos/tests/sssd.nix

···
       start_all()
       machine.wait_for_unit("multi-user.target")
       machine.wait_for_unit("sssd.service")
+      machine.succeed("sssctl config-check")
     '';
 })