Test migration to jest by hand

Apparently using regular expressions is way easier.

test/promisify/promisify.js

@@ -0,0 +1,5 @@
+const promisify = func => (...args) =>
+new Promise((resolve, reject) =>
+func(...args, (err, result) => (err ? reject(err) : resolve(result)))
+);
+module.exports = promisify;