tests/wireguard: init wg-quick

- Duplicate and adapt the existing default wireguard test for wg-quick
- Pull common setup code out into a seperate file

nixos/tests/wireguard/make-peer.nix

@@ -0,0 +1,23 @@
+{ lib, ... }: { ip4, ip6, extraConfig }:
+lib.mkMerge [
+  {
+    boot.kernel.sysctl = {
+      "net.ipv6.conf.all.forwarding" = "1";
+      "net.ipv6.conf.default.forwarding" = "1";
+      "net.ipv4.ip_forward" = "1";
+    };
+
+    networking.useDHCP = false;
+    networking.interfaces.eth1 = {
+      ipv4.addresses = [{
+        address = ip4;
+        prefixLength = 24;
+      }];
+      ipv6.addresses = [{
+        address = ip6;
+        prefixLength = 64;
+      }];
+    };
+  }
+  extraConfig
+]