Improve spec coverage for models/announcement class (#28350)

spec/fabricators/custom_emoji_fabricator.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 Fabricator(:custom_emoji) do
-  shortcode 'coolcat'
+  shortcode { sequence(:shortcode) { |i| "code_#{i}" } }
   domain    nil
   image     { Rails.root.join('spec', 'fixtures', 'files', 'emojo.png').open }
 end