python311Packages.pypng,python312Packages.pypng: fix test imports and PATH

pkgs/development/python-modules/pypng/default.nix

···
     setuptools
   ];
 
-  pythonImportsCheck = [ "png" ];
+  patches = [
+    # pngsuite is imported by code/test_png.py but is not defined in
+    # setup.cfg, so it isn't built - this adds it to py_modules
+    ./setup-cfg-pngsuite.patch
+  ];
+
+  # allow tests to use the binaries produced by this package
+  preCheck = ''
+    export PATH="$out/bin:$PATH"
+  '';
+
+  pythonImportsCheck = [ "png" "pngsuite" ];
 
   nativeCheckInputs = [ pytestCheckHook ];
 

pkgs/development/python-modules/pypng/setup-cfg-pngsuite.patch

···
+diff --git a/setup.cfg b/setup.cfg
+index 04bba8a..db159d2 100644
+--- a/setup.cfg
++++ b/setup.cfg
+@@ -28,6 +28,7 @@ package_dir =
+        = code
+ py_modules =
+     png
++    pngsuite
+ scripts =
+     code/prichunkpng
+     code/pricolpng