Updated the test system

test/primes.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {primes} = require('./_30s.js');
+
+test('primes is a Function', () => {
+  expect(primes).toBeInstanceOf(Function);
+});
+test('Generates primes up to a given number, using the Sieve of Eratosthenes.', () => {
+  expect(primes(10)).toEqual([2, 3, 5, 7]);
+});