[commtest] use registered UserIdentity in backup tests

Summary:
ENG-6145.

Later, we'll need to get userID from Identity to download backup, to make it work we need to use existing user instead of mocks.

Depends on D13803

Test Plan: Run commtest

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13805