Updated the test system

test/getDaysDiffBetweenDates.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {getDaysDiffBetweenDates} = require('./_30s.js');
+
+test('getDaysDiffBetweenDates is a Function', () => {
+  expect(getDaysDiffBetweenDates).toBeInstanceOf(Function);
+});
+test('Returns the difference in days between two dates', () => {
+  expect(getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22'))).toBe(9);
+});