[commtest] Add test for UpdateDeviceList RPC

Summary:
Added integration test that tests D10799.
Reorganized some code to be reusable.

Depends on D10799

Test Plan: Ran both device list integration tests.

Reviewers: varun, will, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10800