remove mkdirp use

node 10.12 has { recursive: true }

src/test/common.js

@@ -1,7 +1,6 @@
 'use strict';
 
 var fs = require('fs'),
-    mkdirp = require('mkdirp'),
     path = require('path'),
     rimraf = require('rimraf');
 
@@ -11,7 +10,7 @@ exports = module.exports = {
 
 function createTree(root, obj) {
     rimraf.sync(root);
-    mkdirp.sync(root);
+    fs.mkdirSync(root, { recursive: true });
 
     function createSubTree(tree, curpath) {
         for (var key in tree) {