Refactor mountComponent to mount

Part of the epic https://gitlab.com/groups/gitlab-org/-/epics/5102

spec/frontend/diffs/components/file_row_stats_spec.js

-import Vue from 'vue';
-import mountComponent from 'helpers/vue_mount_component_helper';
+import { mount } from '@vue/test-utils';
 import FileRowStats from '~/diffs/components/file_row_stats.vue';
 
 describe('Diff file row stats', () => {
-  let Component;
-  let vm;
-
-  beforeAll(() => {
-    Component = Vue.extend(FileRowStats);
-  });
-
-  beforeEach(() => {
-    vm = mountComponent(Component, {
+  const wrapper = mount(FileRowStats, {
+    propsData: {
       file: {
         addedLines: 20,
         removedLines: 10,
       },
-    });
-  });
-
-  afterEach(() => {
-    vm.$destroy();
+    },
   });
 
   it('renders added lines count', () => {
-    expect(vm.$el.querySelector('.cgreen').textContent).toContain('+20');
+    expect(wrapper.find('.cgreen').text()).toContain('+20');
   });
 
   it('renders removed lines count', () => {
-    expect(vm.$el.querySelector('.cred').textContent).toContain('-10');
+    expect(wrapper.find('.cred').text()).toContain('-10');
   });
 });