Updated the test system

test/sum.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {sum} = require('./_30s.js');
+
+test('sum is a Function', () => {
+  expect(sum).toBeInstanceOf(Function);
+});
+test('Returns the sum of two or more numbers/arrays.', () => {
+  expect(sum(...[1, 2, 3, 4])).toBe(10);
+});