See https://github.com/w3c/web-platform-tests/tree/master/encoding. Given that we already have a polyfill for testharness.js it should not be entirely difficult. However, care must be taken to comment out the portions of WPT that either only run in browsers, or are not currently supported by Node.js ('iso-8859-16' encoding), or are not supported by Node.js built with small-icu.