rsyslog: add nixos tests to passthru.tests

ofborg does not build them yet, but at least nixpkgs-update does.
https://github.com/ryantm/nixpkgs-update/pull/118
Note this test passes and would not have detected
https://github.com/NixOS/nixpkgs/issues/79304

pkgs/tools/system/rsyslog/default.nix

@@ -5,6 +5,7 @@
 , liblogging ? null, libnet ? null, hadoop ? null, rdkafka ? null
 , libmongo-client ? null, czmq ? null, rabbitmq-c ? null, hiredis ? null, mongoc ? null
 , libmaxminddb ? null
+, nixosTests ? null
 }:
 
 with stdenv.lib;
@@ -103,6 +104,10 @@ stdenv.mkDerivation rec {
     (mkFlag true                      "generate-man-pages")
   ];
 
+  passthru.tests = {
+    nixos-rsyslogd = nixosTests.rsyslogd;
+  };
+
   meta = {
     homepage = https://www.rsyslog.com/;
     description = "Enhanced syslog implementation";