fix(unit-mocha): node externals are unnecessary for testing client code

close #1548

Author: yyx990803

packages/@vue/cli-plugin-unit-mocha/index.js

@@ -3,11 +3,7 @@ module.exports = api => {
     if (process.env.NODE_ENV === 'test') {
       webpackConfig.merge({
         target: 'node',
-        devtool: 'inline-cheap-module-source-map',
-        externals: [
-          require('webpack-node-externals')(),
-          'vue-server-renderer'
-        ]
+        devtool: 'inline-cheap-module-source-map'
       })
 
       // when target === 'node', vue-loader will attempt to generate

packages/@vue/cli-plugin-unit-mocha/package.json

@@ -25,8 +25,7 @@
     "jsdom": "^11.10.0",
     "jsdom-global": "^3.0.2",
     "mocha": "^5.1.1",
-    "mocha-webpack": "^2.0.0-beta.0",
-    "webpack-node-externals": "^1.7.2"
+    "mocha-webpack": "^2.0.0-beta.0"
   },
   "devDependencies": {
     "@vue/test-utils": "^1.0.0-beta.16",