[commtest] Rename SigningCapableAccount to MockOlmAccount

Summary:
Depends on D12343

Just a rename. I think this name is more accurate to what it's emulating, in context of our architecture.

Test Plan: Commtest

Reviewers: kamil, will, varun, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12344