run tdd, update test cases for flatten.

test/flatten/flatten.js

@@ -1 +1,4 @@
-module.exports = flatten = arr => [].concat(...arr);
+module.exports = flatten = (arr, depth = 1) =>
+depth != 1
+? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flatten(v, depth - 1) : v), [])
+: arr.reduce((a, v) => a.concat(v), []);