graphviz: add reverse deps to passthru.tests

taken from the packages mentioned in the previous bump PR 168502

pkgs/tools/graphics/graphviz/default.nix

@@ -19,6 +19,8 @@
 , xorg
 , ApplicationServices
 , python3
+, fltk
+, exiv2
 , withXorg ? true
 }:
 
@@ -93,6 +95,11 @@ stdenv.mkDerivation {
       --replace /usr/bin/vimdot $out/bin/vimdot \
   '';
 
+  passthru.tests = {
+    inherit (python3.pkgs) pygraphviz;
+    inherit fltk exiv2;
+  };
+
   meta = with lib; {
     homepage = "https://graphviz.org";
     description = "Graph visualization tools";