Updated the test system

test/tail.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {tail} = require('./_30s.js');
+
+test('tail is a Function', () => {
+  expect(tail).toBeInstanceOf(Function);
+});
+test('Returns tail', () => {
+  expect(tail([1, 2, 3])).toEqual([2, 3]);
+});
+test('Returns tail', () => {
+  expect(tail([1])).toEqual([1]);
+});