Simplify creating notifications in specs

spec/factories/notifications.rb

@@ -6,6 +6,18 @@ FactoryBot.define do
     trait :read do
       read_at { Time.current }
     end
+
+    trait :for_proposal_notification do
+      association :notifiable, factory: :proposal_notification
+    end
+
+    trait :for_comment do
+      association :notifiable, factory: :comment
+    end
+
+    trait :for_poll_question do
+      association :notifiable, factory: :poll_question
+    end
   end
 
   factory :admin_notification do