Add test for N+1 queries

spec/lib/gitlab/background_migration/populate_finding_uuid_for_vulnerability_feedback_spec.rb

@@ -86,5 +86,10 @@ RSpec.describe Gitlab::BackgroundMigration::PopulateFindingUuidForVulnerabilityF
 
       expect(Gitlab::BackgroundMigration::Logger).to have_received(:info).once
     end
+
+    it 'preloads the finding and identifier records to prevent N+1 queries' do
+      # Load feedback records(1), load findings(2), load identifiers(3) and finally update feedback records one by one(6)
+      expect { populate_finding_uuids }.not_to exceed_query_limit(6)
+    end
   end
 end