Switch to shallowMountExtended

Refactors merge-conflict-resolver-app component's spec to use
shallowMountExtended and its helpers instead of the default
shallowMount.

spec/frontend/merge_conflicts/components/merge_conflict_resolver_app_spec.js

 import { GlSprintf } from '@gitlab/ui';
-import { shallowMount } from '@vue/test-utils';
 import Vue, { nextTick } from 'vue';
 import Vuex from 'vuex';
+import { shallowMountExtended } from 'helpers/vue_test_utils_helper';
 import InlineConflictLines from '~/merge_conflicts/components/inline_conflict_lines.vue';
 import ParallelConflictLines from '~/merge_conflicts/components/parallel_conflict_lines.vue';
 import component from '~/merge_conflicts/merge_conflict_resolver_app.vue';
@@ -18,7 +18,7 @@ describe('Merge Conflict Resolver App', () => {
   const decoratedMockFiles = decorateFiles(conflictsMock.files);
 
   const mountComponent = () => {
-    wrapper = shallowMount(component, {
+    wrapper = shallowMountExtended(component, {
       store,
       stubs: { GlSprintf },
       provide() {
@@ -41,16 +41,16 @@ describe('Merge Conflict Resolver App', () => {
     wrapper.destroy();
   });
 
-  const findLoadingSpinner = () => wrapper.find('[data-testid="loading-spinner"]');
-  const findConflictsCount = () => wrapper.find('[data-testid="conflicts-count"]');
-  const findFiles = () => wrapper.findAll('[data-testid="files"]');
+  const findLoadingSpinner = () => wrapper.findByTestId('loading-spinner');
+  const findConflictsCount = () => wrapper.findByTestId('conflicts-count');
+  const findFiles = () => wrapper.findAllByTestId('files');
   const findFileHeader = (w = wrapper) => w.find('[data-testid="file-name"]');
   const findFileInteractiveButton = (w = wrapper) => w.find('[data-testid="interactive-button"]');
   const findFileInlineButton = (w = wrapper) => w.find('[data-testid="inline-button"]');
-  const findSideBySideButton = () => wrapper.find('[data-testid="side-by-side"]');
+  const findSideBySideButton = () => wrapper.findByTestId('side-by-side');
   const findInlineConflictLines = (w = wrapper) => w.find(InlineConflictLines);
   const findParallelConflictLines = (w = wrapper) => w.find(ParallelConflictLines);
-  const findCommitMessageTextarea = () => wrapper.find('[data-testid="commit-message"]');
+  const findCommitMessageTextarea = () => wrapper.findByTestId('commit-message');
 
   it('shows the amount of conflicts', () => {
     mountComponent();