Updated the test system

test/percentile.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {percentile} = require('./_30s.js');
+
+test('percentile is a Function', () => {
+  expect(percentile).toBeInstanceOf(Function);
+});
+test('Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.', () => {
+  expect(percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6)).toBe(55);
+});