[commtest] Check log logic in backup tests

Summary:
commtest part of ENG-5867 : Final testing tasks

Depends on D10533

Used the functions implemented in the previous diffs to update the backup tests. Tested:

• more logs than the log page size
• both logs that are saved in dynamodb and in blob service

Test Plan: Run the test

Reviewers: bartek, kamil

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10534