Use AVA 4 for the self-hosted tests

Author: novemberborn

ava.config.cjs renamed to ava.config.js

@@ -1,7 +1,4 @@
-// 👉 Due to the package exports, XO's use of our ESLint plugin loads this
-// file using the AVA code in the repository, but our self-hosted tests use the
-// installed "test-ava" version.
-module.exports = {
+export default { // eslint-disable-line import/no-anonymous-default-export
 	files: ['test/**', '!test/**/{fixtures,helpers}/**'],
 	ignoredByWatcher: ['{coverage,docs,media,test-d,test-tap}/**'],
 	environmentVariables: {