cups: add 'cups-pdf' vm test to passthru.tests

While aimed at the cups-pdf printing package,
this vm tests also serves as a test for
the cups printing module in general.

pkgs/misc/cups/default.nix

@@ -138,6 +138,7 @@ stdenv.mkDerivation rec {
 
   passthru.tests = {
     inherit (nixosTests)
+      cups-pdf
       printing-service
       printing-socket
     ;