Bug 1543538 fix TV failure in browser_ext_tabs_newtab_private.js due to async test r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D27015

--HG--
extra : moz-landing-system : lando

browser/components/extensions/test/browser/browser_ext_tabs_newtab_private.js

@@ -73,7 +73,7 @@ add_task(async function test_new_tab_private() {
 
   verifyPrefSettings(true, false);
 
-  promiseUpdatePrivatePermission(true, extension);
+  await promiseUpdatePrivatePermission(true, extension);
 
   await extension.unload();
 });