[services] Tunnelbroker - Database item tests initialization

Summary:
Database (DynamoDB) item tests initialization.

Related linear task: ENG-686

Test Plan: Run yarn test-tunnelbroker-service all tests are run and succeed.

Reviewers: karol-bisztyga, palys-swm, varun

Reviewed By: karol-bisztyga, palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3643