Updated the test system

test/deepFlatten.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {deepFlatten} = require('./_30s.js');
+
+test('deepFlatten is a Function', () => {
+  expect(deepFlatten).toBeInstanceOf(Function);
+});
+test('Deep flattens an array', () => {
+  expect(deepFlatten([1, [2], [[3], 4], 5])).toEqual([1, 2, 3, 4, 5]);
+});