Test migration to jest by hand

Apparently using regular expressions is way easier.

test/reduceWhich/reduceWhich.js

@@ -0,0 +1,3 @@
+const reduceWhich = (arr, comparator = (a, b) => a - b) =>
+arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));
+module.exports = reduceWhich;