Fix notifiable in-app topic specs

These specs had never been tested before and were always broken.

spec/factories/classifications.rb

@@ -21,6 +21,12 @@ FactoryBot.define do
     sequence(:title) { |n| "Topic title #{n}" }
     sequence(:description) { |n| "Description as comment #{n}" }
     association :author, factory: :user
+
+    trait :with_community do
+      community { create(:proposal).community }
+    end
+
+    factory :topic_with_community, traits: [:with_community]
   end
 
   factory :related_content do

spec/features/topics_spec.rb

@@ -3,7 +3,7 @@ require "rails_helper"
 describe "Topics" do
 
   context "Concerns" do
-    it_behaves_like "notifiable in-app", Topic
+    it_behaves_like "notifiable in-app", :topic_with_community
   end
 
   context "New" do