gobjectIntrospection: test our own patches

We patch gobject-introspection somewhat heavily, so add some unit tests that
will catch potential errors when we're eg. updating the patches.

There's two tests:

- Basic test for absolute nix store paths (testing absolute_shlib_path.patch)
- Test for macos `@rpath` sanitizing, including
  fallback_libpath (macos-shared-library.patch)

pkgs/development/libraries/gobject-introspection/default.nix

@@ -41,6 +41,10 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./macos-shared-library.patch
+    (substituteAll {
+      src = ./test_shlibs.patch;
+      inherit nixStoreDir;
+    })
     (substituteAll {
       src = ./absolute_shlib_path.patch;
       inherit nixStoreDir;