knot-dns: add passthru.tests

I hope I got the conventions right (found in doc/stdenv/meta.chapter.md)

pkgs/servers/dns/knot-dns/default.nix

@@ -1,6 +1,6 @@
 { lib, stdenv, fetchurl, pkg-config, gnutls, liburcu, lmdb, libcap_ng, libidn2, libunistring
 , systemd, nettle, libedit, zlib, libiconv, libintl, libmaxminddb, libbpf, nghttp2
-, autoreconfHook
+, autoreconfHook, nixosTests
 }:
 
 let inherit (lib) optional optionals; in
@@ -56,6 +56,8 @@ stdenv.mkDerivation rec {
     rm -r "$out"/lib/*.la
   '';
 
+  passthru.tests = { inherit (nixosTests) knot; };
+
   meta = with lib; {
     description = "Authoritative-only DNS server from .cz domain registry";
     homepage = "https://knot-dns.cz";