Updated the test system

test/reduceSuccessive.test.js

@@ -0,0 +1,17 @@
+const expect = require('expect');
+const {reduceSuccessive} = require('./_30s.js');
+
+test('reduceSuccessive is a Function', () => {
+  expect(reduceSuccessive).toBeInstanceOf(Function);
+});
+test('Returns the array of successively reduced values', () => {
+  expect(reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0)).toEqual([
+    0,
+    1,
+    3,
+    6,
+    10,
+    15,
+    21
+  ]);
+});