Updated the test system

test/symmetricDifference.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {symmetricDifference} = require('./_30s.js');
+
+test('symmetricDifference is a Function', () => {
+  expect(symmetricDifference).toBeInstanceOf(Function);
+});
+test('Returns the symmetric difference between two arrays.', () => {
+  expect(symmetricDifference([1, 2, 3], [1, 2, 4])).toEqual([3, 4]);
+});
+test('Returns duplicates from one array', () => {
+  expect(symmetricDifference([1, 2, 2], [1, 3, 1])).toEqual([2, 2, 3]);
+});