Updated the test system

test/powerset.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {powerset} = require('./_30s.js');
+
+test('powerset is a Function', () => {
+  expect(powerset).toBeInstanceOf(Function);
+});
+test('Returns the powerset of a given array of numbers.', () => {
+  expect(powerset([1, 2])).toEqual([[], [1], [2], [2, 1]]);
+});