Don't click button until page JS loaded

We add `.page-initialised` at the end of `deferredInitialisation`,
which is where the userCallbacks are set up

Better way is to disable the inactive button in haml then enable in JS
but
that has some other UI considerations (disabled tooltip while loading)

ee/spec/features/admin/licenses/show_user_count_threshold_spec.rb

@@ -56,7 +56,7 @@ RSpec.describe 'Display approaching user count limit banner', :js do
         context 'when banner was dismissed' do
           before do
             visit admin_root_path
-            find('[data-testid="gitlab-ee-license-banner-dismiss"]').click
+            find('body.page-initialised [data-testid="gitlab-ee-license-banner-dismiss"]').click
           end
 
           it_behaves_like 'a hidden banner'