potrace: 1.15 -> 1.16 (#69748)

* potrace: 1.15 -> 1.16

Semi-automatic update generated by
https://github.com/ryantm/nixpkgs-update tools. This update was made
based on information from
https://repology.org/metapackage/potrace/versions

* potrace: enable tests

Upstream provides a test suite with make check