Test migration to jest by hand

Apparently using regular expressions is way easier.

test/intersectionBy/intersectionBy.js

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