[Tunnelbroker] Add integration tests

Summary:
For some features, it's easier to just have a unittest demonstrate
the expected behavior. This sets up that framework, and modifies
the existing code to make it more explicit as to what is going on
when debugging.

Depends on D7659

Test Plan:

(cd services/tunnelbroker && RUST_LOG=debug cargo run &)
cd services/commtest && cargo test --test tunnelbroker_integration_test

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7661