[commtest] Restructure code into olm_account module

Summary:
Depends on D12344

Moved some code:

• renamed olm_account_infos.rs into olm_account.rs
• moved MockOlmAccount to this module, so we have both ClientPublicKeys and MockOlmAccount in one place

Test Plan: Commtest

Reviewers: kamil, will, varun, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12345