Test migration to jest by hand

Apparently using regular expressions is way easier.

test/flatten/flatten.js

@@ -0,0 +1,3 @@
+const flatten = (arr, depth = 1) =>
+arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);
+module.exports = flatten;