Updated the test system

test/findLastIndex.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {findLastIndex} = require('./_30s.js');
+
+test('findLastIndex is a Function', () => {
+  expect(findLastIndex).toBeInstanceOf(Function);
+});
+test('Finds last index for which the given function returns true', () => {
+  expect(findLastIndex([1, 2, 3, 4], n => n % 2 === 1)).toBe(2);
+});