[Commtest] introduce function for sending WebSocket message to Tunnelbroker

Summary:
This is a refactor which will make writing tests easier.

Depends on D9467

Test Plan: Run tests to make sure everything still works

Reviewers: michal, bartek, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9468