nixosTests.etcd-cluster: update name

nixos/tests/etcd-cluster.nix

···
     networking.firewall.allowedTCPPorts = [ 2380 ];
   };
 in {
-  name = "etcd";
+  name = "etcd-cluster";
 
   meta = with pkgs.lib.maintainers; {
     maintainers = [ offline ];