[services] Backup - Add DB tests for Logs

Summary:
Add tests for database operations for LogItems

Depends on D3175

Test Plan:

cd services
yarn test-backup-service

Reviewers: palys-swm, geekbrother, varun, ashoat, jimpo

Reviewed By: palys-swm, ashoat

Subscribers: benschac, ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D3176