Fix broken CI job in single database mode

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/184021
introduced a test that assumed a decomposed database were used, but we
have `single-db` pipelines that only use one database. Adjust the spec
to handle both single and multiple databases accordingly.

spec/keeps/overdue_finalize_background_migration_spec.rb

@@ -124,7 +124,7 @@
       let(:gitlab_schema) { 'gitlab_ci' }
 
       it 'returns the database name' do
-        expect(database_name).to eq('ci')
+        expect(database_name).to eq(database_exists?(:ci) ? 'ci' : 'main')
       end
     end
   end