Test migration to jest by hand

Apparently using regular expressions is way easier.

test/differenceBy/differenceBy.js

@@ -0,0 +1,5 @@
+const differenceBy = (a, b, fn) => {
+const s = new Set(b.map(v => fn(v)));
+return a.filter(x => !s.has(fn(x)));
+};
+module.exports = differenceBy;