nixos: add test for sslh

pkgs/servers/sslh/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, libcap, libconfig, perl, tcp_wrappers, pcre }:
+{ stdenv, fetchurl, libcap, libconfig, perl, tcp_wrappers, pcre, nixosTests }:
 
 stdenv.mkDerivation rec {
   pname = "sslh";
@@ -19,6 +19,10 @@ stdenv.mkDerivation rec {
 
   hardeningDisable = [ "format" ];
 
+  passthru.tests = {
+    inherit (nixosTests) sslh;
+  };
+
   meta = with stdenv.lib; {
     description = "Applicative Protocol Multiplexer (e.g. share SSH and HTTPS on the same port)";
     license = licenses.gpl2Plus;