[commtest] refactor backup service integration tests

Summary:
Part of ENG-6145.

Tracked in ENG-9650.

1. Create mod for utils, similarly to e.g. blob_utils
2. Reduces code duplication and cleanup
3. Move code to a place so it can be used in more places (later in the stack I'll add a lot more backup integration tests)

Depends on D13809

Test Plan: run commtest

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13872