fix(mocha): workaround the ShadowRoot issue in Vue 3.0.5 (#6186)

Author: haoqunjiang

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

@@ -2,3 +2,5 @@ require('jsdom-global')(undefined, { pretendToBeVisual: true, url: 'http://local
 
 // https://github.com/vuejs/vue-test-utils/issues/936
 window.Date = Date
+// https://github.com/vuejs/vue-next/pull/2943
+global.ShadowRoot = window.ShadowRoot