Multiselect integration test

Testing of #111 uncovers that protocol negotiation happens more often than in the master branch. Moreover, that negotiation often fails due to unexpected messages which may indicate the error-prone messages flow and order.

The issue is to create some integration test for the multiselect protocol to reveal such errors and verify messages order correctness without any dependency on underlying implementation (either callbacks or tasks scheduling - #111 addresses it).

Log example of multiselect issues when #111 gets employed:

[2021-03-15 15:29:45.873680000][th:584649][info] multiselect got a unexpected request-response combination - sending 'ls'
[2021-03-15 15:29:45.873701000][th:584649][warning] listener_mgr can not negotiate protocols, other side has violated a protocol and sent an unexpected message
[2021-03-15 15:29:45.984183000][th:584649][info] multiselect got a unexpected request-response combination - sending 'ls'
[2021-03-15 15:29:45.984203000][th:584649][warning] listener_mgr can not negotiate protocols, other side has violated a protocol and sent an unexpected message

Revisa el issue #111 junto con el flujo del protocolo multiselect y cualquier punto de entrada existente para las pruebas de integración. Reproduce el mensaje inesperado registrado y los fallos en el orden de negociación; después, define una prueba que verifique el orden de los mensajes sin depender de callbacks ni de la planificación de tareas.