Write extra PublisherSubscriberTest for up-transport-zenoh-cpp

[gregmedd]

“Extra” tests in a transport library provide an opportunity to exercise end-to-end layer communication. Layer 2 tests do not contribute to coverage for transport libraries since transport implementations are entirely layer 1. However, layer 2 tests are critical in evaluating the health and performance of a transport implementation.

This test will validate the interoperability between Publisher and Subscriber over Zenoh. It should verify that published messages arrive with all data intact and are received only by the intended subscribers. Additionally, it should verify, to the extent possible, that messages can be exchanged between threads, processes, and across the (loopback) network. Where possible, it should also make assertions about performance and validate message handling under load.