Add spec for user_id uniq in NotificationSetting model

spec/models/notification_setting_spec.rb

@@ -7,10 +7,11 @@ RSpec.describe NotificationSetting, type: :model do
   end
 
   describe "Validation" do
-    subject { NotificationSetting.new }
+    subject { NotificationSetting.new(source_id: 1, source_type: 'Project') }
 
     it { is_expected.to validate_presence_of(:user) }
     it { is_expected.to validate_presence_of(:source) }
     it { is_expected.to validate_presence_of(:level) }
+    it { is_expected.to validate_uniqueness_of(:user_id).scoped_to([:source_id, :source_type]).with_message(/already exists in source/) }
   end
 end