[commtest][identity] Add test for FindUserID RPC

Summary:
Added integration test for FindUserID RPC added in D9834.

Depends on D9834

Test Plan: The test passes locally and on CI

Reviewers: kamil, michal, varun, wyilio

Reviewed By: varun

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9835