Add test_duplex.cpp with 20 full-duplex two-instance tests

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>
Agent-Logs-Url: https://github.com/petesramek/tiny-link/sessions/4d4c519e-c6b6-4c49-b432-9e235e3ca403

Author: Copilot