[lib] Add unit tests for device list verification

Summary:
Added unit tests for device list verification function introduced in D11929.
Added helpers to mock identityClient.getDeviceListHistoryForUser() and olmAPI.verifyMessage().

Depends on D11929

Test Plan: unit tests pass

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11930