Fix flaky content_editor_spec

See https://gitlab.com/gitlab-org/gitlab/-/issues/338553

qa/qa/page/component/wiki_page_form.rb

@@ -47,6 +47,7 @@ module QA
           within_element(:try_new_editor_container) do
             click_button('Use the new editor')
           end
+          has_element?(:content_editor_container)
         end
       end
     end