[commtest] Fix flaky identity-tunnelbroker test

Summary:
This test stops working when I introduced random and unique device IDs.

What was wrong:

1. It was using register_user_device(None, None) which always created a user with the same (constant) device ID. Tunnelbroker was treating this as it was always the same device. So multiple users were treated as one recipient.
2. When fixed this, it turns out that RefreshKeysRequest is expected to be sent twice, but recently it was fixed to be sent only once. Previous behavior was mentioned in https://phab.comm.dev/D9577?id=32347#inline-60330 It was working before only because other tests also sent refresh requests that were enqueued in Tunnelbroker for this constant device ID.

Test Plan: Commtest - before and after random device IDs

Reviewers: kamil, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12346