Updated the test system

test/symmetricDifferenceBy.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {symmetricDifferenceBy} = require('./_30s.js');
+
+test('symmetricDifferenceBy is a Function', () => {
+  expect(symmetricDifferenceBy).toBeInstanceOf(Function);
+});
+test('Returns the symmetric difference between two arrays, after applying the provided function to each array element of both', () => {
+  expect(symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor)).toEqual([1.2, 3.4]);
+});