Updated the test system

test/reverseString.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {reverseString} = require('./_30s.js');
+
+test('reverseString is a Function', () => {
+  expect(reverseString).toBeInstanceOf(Function);
+});
+test('Reverses a string.', () => {
+  expect(reverseString('foobar')).toBe('raboof');
+});