generate new test that did already exist & update all module snippets with declaration

test/groupBy/groupBy.js

@@ -1,4 +1,4 @@
-module.exports = (arr, func) =>
+module.exports = groupBy = (arr, func) =>
 arr.map(typeof func === 'function' ? func : val => val[func]).reduce((acc, val, i) => {
 acc[val] = (acc[val] || []).concat(arr[i]);
 return acc;