Fix tests broken due to the firewall being enabled by default

nixos/tests/proxy.nix

@@ -5,12 +5,10 @@ let
   backend =
     { config, pkgs, ... }:
 
-    {
-      services.openssh.enable = true;
-
-      services.httpd.enable = true;
+    { services.httpd.enable = true;
       services.httpd.adminAddr = "foo@example.org";
       services.httpd.documentRoot = "${pkgs.valgrind}/share/doc/valgrind/html";
+      networking.firewall.allowedTCPPorts = [ 80 ];
     };
 
 in
@@ -21,8 +19,7 @@ in
     { proxy =
         { config, pkgs, nodes, ... }:
 
-        {
-          services.httpd.enable = true;
+        { services.httpd.enable = true;
           services.httpd.adminAddr = "bar@example.org";
           services.httpd.extraModules = ["proxy_balancer"];
 
@@ -50,6 +47,8 @@ in
               # For testing; don't want to wait forever for dead backend servers.
               ProxyTimeout      5
             '';
+
+          networking.firewall.allowedTCPPorts = [ 80 ];
         };
 
       backend1 = backend;