fix: eliminate calling deprecated function in cli-plugin-e2e-cypress and cli-plugin-e2e-nightwatch

Due to webpack-dev-server's close() method became deprecated in version v4.0.0, currently vue-cli
is using version ^4.7.3, everytime when cli-plugin-e2e-cypress or cli-plugin-e2e-nightwatch ran any
tests a deprecation warning appeared. From now on method stop() is called instead of close() as
advised by the deprecation warning.

close #7157

Author: blzsaa

packages/@vue/cli-plugin-e2e-cypress/index.js

@@ -36,8 +36,8 @@ module.exports = (api, options) => {
       resolveModule('cypress/bin/cypress', __dirname)
     const runner = execa(cypressBinPath, cyArgs, { stdio: 'inherit' })
     if (server) {
-      runner.on('exit', () => server.close())
-      runner.on('error', () => server.close())
+      runner.on('exit', () => server.stop())
+      runner.on('error', () => server.stop())
     }
 
     if (process.env.VUE_CLI_TEST) {

packages/@vue/cli-plugin-e2e-nightwatch/index.js

@@ -80,8 +80,8 @@ module.exports = (api, options) => {
       const nightWatchBinPath = require.resolve('nightwatch/bin/nightwatch')
       const runner = execa(nightWatchBinPath, rawArgs, { stdio: 'inherit' })
       if (server) {
-        runner.on('exit', () => server.close())
-        runner.on('error', () => server.close())
+        runner.on('exit', () => server.stop())
+        runner.on('error', () => server.stop())
       }
 
       if (process.env.VUE_CLI_TEST) {