Migrate CE MrWidgetOptions spec to @vue/test-utils

This is a mostly-straight-forward and nearly minimal migration; there
are a lot of issues with the existing suite, e.g., mutating props
directly; making assertions against component internals; inconsistent
use of async/done/promise chains. Fixing these problems is well outside
the scope of this, and can be addressed later.

Partly addresses https://gitlab.com/gitlab-org/gitlab/-/issues/220476.

app/assets/javascripts/vue_merge_request_widget/index.js

@@ -26,7 +26,11 @@ export default () => {
 
   registerExtension(issueExtension);
 
-  const vm = new Vue({ ...MrWidgetOptions, apolloProvider });
+  const vm = new Vue({
+    el: '#js-vue-mr-widget',
+    ...MrWidgetOptions,
+    apolloProvider,
+  });
 
   window.gl.mrWidget = {
     checkStatus: vm.checkStatus,

app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue

@@ -48,7 +48,6 @@ import GroupedAccessibilityReportsApp from '../reports/accessibility_report/grou
 import getStateQuery from './queries/get_state.query.graphql';
 
 export default {
-  el: '#js-vue-mr-widget',
   // False positive i18n lint: https://gitlab.com/gitlab-org/frontend/eslint-plugin-i18n/issues/25
   // eslint-disable-next-line @gitlab/require-i18n-strings
   name: 'MRWidget',