WebUI NTP: enable app test

The https://chromium-review.googlesource.com/c/chromium/src/+/2076533 CL
makes it so the promo is not loaded on the app page during tests. This
is likely the cause of the flaky tests.

Bug: 1056063
Change-Id: I86c8e17e13625a444e278e9d5f0fe78b78ce3cbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083339
Commit-Queue: Esmael Elmoslimany <aee@chromium.org>
Auto-Submit: Esmael Elmoslimany <aee@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746077}

chrome/test/data/webui/new_tab_page/new_tab_page_browsertest.js

@@ -35,8 +35,7 @@ var NewTabPageAppTest = class extends NewTabPageBrowserTest {
   }
 };
 
-// This test is flaky, https://crbug.com/1056063
-TEST_F('NewTabPageAppTest', 'DISABLED_All', function() {
+TEST_F('NewTabPageAppTest', 'All', function() {
   mocha.run();
 });