[services] Tunnelbroker - Database SessionItem tests

Summary:
Database (DynamoDB) item tests. For the SessionItem database entity we perform:

• Check if the table is available,
• Create and fulfill database item entity,
• Put into the database table,
• Find the record using the primary key,
• Compare record fields of created item and item from database record,
• Remove the testing record from the database.

Related linear task: ENG-686

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

Reviewers: varun, palys-swm, karol-bisztyga

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3686