nginx: add link to acme nixos test

pkgs/servers/http/nginx/generic.nix

···
     tests = {
       inherit (nixosTests) nginx nginx-auth nginx-etag nginx-pubhtml nginx-sandbox nginx-sso;
       variants = lib.recurseIntoAttrs nixosTests.nginx-variants;
+      acme-integration = nixosTests.acme;
     };
   };