Updated the test system

test/btoa.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {btoa} = require('./_30s.js');
+
+test('btoa is a Function', () => {
+  expect(btoa).toBeInstanceOf(Function);
+});
+test('btoa("foobar") equals "Zm9vYmFy"', () => {
+  expect(btoa('foobar')).toBe('Zm9vYmFy');
+});