[services] Tests - Update backup test

Summary:
Depends on D4324

linear task: https://linear.app/comm/issue/ENG-1288/properly-handle-the-dynamodb-item-size-limit

I added a case when we first add an item, it is stored in the database, then we add so many attachment holders, that it goes beyond the dynamo limit and we move this data to the S3.

Test Plan:

cd services
yarn run-integration-tests backup

Reviewers: palys-swm, varun, ashoat

Reviewed By: palys-swm

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

Differential Revision: https://phab.comm.dev/D4326