test($core): use es6 import

ref: https://github.com/facebook/jest/issues/6835

.babelrc

-{
-  "env": {
-    "test": {
-      "presets": [
-        ["@babel/preset-env", { "targets": { "node": 8 }}]
-      ]
-    }
-  }
-}

babel.config.js

+module.exports = {
+  'env': {
+    'test': {
+      'presets': [
+        ['@babel/preset-env', { 'targets': { 'node': 8 }}]
+      ]
+    }
+  }
+}

packages/@vuepress/core/__test__/plugin-api/Option.spec.js

-// TODO change to ES6 import
-// https://github.com/facebook/jest/issues/6835
-
-const Option = require('../../lib/plugin-api/Option')
+import Option from '../../lib/plugin-api/Option'
 
 describe('Option', () => {
   test('shoould option record the key', () => {

packages/@vuepress/core/__test__/plugin-api/PluginAPI.spec.js

@@ -2,8 +2,8 @@ jest.mock('vuepress-plugin-a')
 jest.mock('vuepress-plugin-b')
 jest.mock('@org/vuepress-plugin-a')
 
-const PluginAPI = require('../../lib/plugin-api/index')
-const { PLUGIN_OPTION_MAP } = require('../../lib/plugin-api/constants')
+import PluginAPI from '../../lib/plugin-api/index'
+import { PLUGIN_OPTION_MAP } from '../../lib/plugin-api/constants'
 
 describe('Plugin', () => {
   test('should \'registerOption\' work.', () => {

packages/@vuepress/core/__test__/plugin-api/PluginUtil.spec.js

 jest.mock('vuepress-plugin-a')
 jest.mock('@org/vuepress-plugin-a')
 
-const {
+import {
   resolvePlugin,
   hydratePlugin,
   resolveScopePackage
-} = require('../../lib/plugin-api/util')
+} from '../../lib/plugin-api/util'
 
 function resolveMockModule (name) {
   return require(`../../../../../__mocks__/${name}`)
 }
 
-// const Plugin = require('../../lib/plugin-api/index')
-
 describe('resolvePlugin', () => {
   test('should resolve scope packages correctly.', () => {
     const pkg1 = resolveScopePackage('@vuepress/plugin-a')