Test migration to jest by hand

Apparently using regular expressions is way easier.

test/unionWith/unionWith.js

@@ -0,0 +1,3 @@
+const unionWith = (a, b, comp) =>
+Array.from(new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)]));
+module.exports = unionWith;