[Tunnelbroker] add authentication tests

Summary:
I implemented those to make sure D8918 works in both cases, so publishing, maybe will be helpful.

Depends on D8918

Test Plan:

• Start identity service, tunnelbroker

cd services/commtest
cargo test --test identity_tunnelbroker_tests

Reviewers: bartek, michal, varun, jon

Reviewed By: bartek, michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9200