[Commtest] extract code to create a socket session with Tunnelbroker

Summary: Minor cleanup, putting duplicated code in one place - helps me with testing.

Test Plan:

1. Run Identity
2. Run Tunnelbroker
3. Go to commtest/
4. Run:

cargo test --test identity_tunnelbroker_tests -- --nocapture

5. Run:

cargo test --test tunnelbroker_integration_test -- --nocapture

Reviewers: michal, bartek, varun, jon

Reviewed By: michal, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9310