Updated the test system

test/flatten.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {flatten} = require('./_30s.js');
+
+test('flatten is a Function', () => {
+  expect(flatten).toBeInstanceOf(Function);
+});
+test('Flattens an array', () => {
+  expect(flatten([1, [2], 3, 4])).toEqual([1, 2, 3, 4]);
+});
+test('Flattens an array', () => {
+  expect(flatten([1, [2, [3, [4, 5], 6], 7], 8], 2)).toEqual([1, 2, 3, [4, 5], 6, 7, 8]);
+});