Add jQuery to jest test_setup

spec/frontend/test_setup.js

 import Vue from 'vue';
 import * as jqueryMatchers from 'custom-jquery-matchers';
+import $ from 'jquery';
 import Translate from '~/vue_shared/translate';
 import axios from '~/lib/utils/axios_utils';
 import { initializeTestTimeout } from './helpers/timeout';
 import { loadHTMLFixture, setHTMLFixture } from './helpers/fixtures';
 
+// Expose jQuery so specs using jQuery plugins can be imported nicely.
+// Here is an issue to explore better alternatives:
+// https://gitlab.com/gitlab-org/gitlab-ee/issues/12448
+window.jQuery = $;
+
 process.on('unhandledRejection', global.promiseRejectionHandler);
 
 afterEach(() =>