generate new test that did already exist & update all module snippets with declaration

test/percentile/percentile.js

@@ -1,2 +1,2 @@
-module.exports = (arr, val) =>
+module.exports = percentile = (arr, val) =>
 100 * arr.reduce((acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0) / arr.length;