Fix flaky admin notifications spec

It was failing when executed right before midnight due to the date
changing between the moment the notification is created and the moment
the test checks the notification shows the current date.

spec/features/admin/admin_notifications_spec.rb

@@ -34,7 +34,7 @@ feature "Admin Notifications" do
   end
 
   context "Index" do
-    scenario "Valid Admin Notifications" do
+    scenario "Valid Admin Notifications", :with_frozen_time do
       draft = create(:admin_notification, segment_recipient: :all_users, title: 'Not yet sent')
       sent = create(:admin_notification, :sent, segment_recipient: :administrators,
                                                 title: 'Sent one')