tests bsim bt: Add basic connection test with HCI UART async controllers

Add a basic connection test between two devices,
in which the controllers are in separate nrf52_bsim devices
connected over UART to the devices running the host+app.
The controllers are running the HCI UART async sample.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>