[Tunnelbroker] add recipient confirmation tests

Summary:
Tests to make sure it works properly and the message is deleted only after confirmation.

Depends on D9577

Test Plan:

1. Run tests
2. Make sure DDB is empty after running tests.

Reviewers: michal, bartek, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9578