dnscrypt-wrapper: link NixOS test

pkgs/tools/networking/dnscrypt-wrapper/default.nix

···
-{ lib, stdenv, fetchFromGitHub, autoreconfHook, pkg-config, libsodium, libevent }:
+{ lib, stdenv, fetchFromGitHub, autoreconfHook, pkg-config, libsodium, libevent, nixosTests }:
 
 stdenv.mkDerivation rec {
   pname = "dnscrypt-wrapper";
···
 
   nativeBuildInputs = [ pkg-config autoreconfHook ];
   buildInputs = [ libsodium libevent ];
+
+  passthru.tests = {
+    inherit (nixosTests) dnscrypt-wrapper;
+  };
 
   meta = with lib; {
     description = "A tool for adding dnscrypt support to any name resolver";