fix: add XMLSerializer to the global scope

In @vue/test-utils 2.0.0rc18, The ussage of XMLSerializer was introduced: https://github.com/vuejs/test-utils/blob/d86cbc333de7b2ec1208f5d3c65e0879671dd01e/src/utils/stringifyNode.ts#L4

XMLSerializer needs to be available as a global so cli-plugin-unit-mocha will stay compatible with newer versions of @vue/test-utils.

Author: maerteijn

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

@@ -6,3 +6,6 @@ window.Date = Date
 global.ShadowRoot = window.ShadowRoot
 
 global.SVGElement = window.SVGElement
+
+// https://github.com/vuejs/test-utils/issues/1253
+global.XMLSerializer = window.XMLSerializer