[Identity/Tunnelbroker] Add integration tests

Summary:
To make the review not as extensive, I split up the integration
tests into its own diff.

Follow up for: https://linear.app/comm/issue/ENG-4446

Depends on D8748

Test Plan:

# Start localstack and rabbitmq
comm-dev services start

(cd services/identity && cargo run -- server)
(cd services/tunnelbroker && cargo run)

cd services/commtest
cargo test --test identity_tunnelbroker_tests

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8749