fix: apply a workaround for the prettier / jsdom-global bug

Since this issue is not going to get a proper fix any time soon,
this workaround is better than pinning prettier version in
`@vue/component-compiler-utils`

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

@@ -1 +1,4 @@
 require('jsdom-global')(undefined, { pretendToBeVisual: true, url: 'http://localhost' })
+
+// https://github.com/vuejs/vue-test-utils/issues/936
+window.Date = Date