[commtest] Add test for device list signing

Summary:
Added integration test to commtest that verifies device list signature verification in Identity UpdateDeviceList RPC.

Also did some small refactors to existing tests to reuse some code.

Depends on D11959

Test Plan: Commtest

Reviewers: will, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11982