Migrated tests to jest

Used jest-codemods to migrate, will have to pass everything by hand before we can merge.

test/mapKeys/mapKeys.js

@@ -1,6 +0,0 @@
-const mapKeys = (obj, fn) =>
-Object.keys(obj).reduce((acc, k) => {
-acc[fn(obj[k], k, obj)] = obj[k];
-return acc;
-}, {});
-module.exports = mapKeys;