perlPackages.JSON: fix tests

pkgs/top-level/perl-packages.nix

···
     preConfigure = ''
       substituteInPlace Makefile.PL --replace "exit 0;" ""
     '';
+    buildInputs = [ TestPod ];
     meta = {
       description = "JSON (JavaScript Object Notation) encoder/decoder";
       license = with stdenv.lib.licenses; [ artistic1 gpl1Plus ];