tests: Bluetooth: Mesh: make msg_mesh tolerant to tx time volatility

msg_mesh bsim test is run in almost all configurations.
The test uses manual polling with hardcoded delays. This PR removes
manual polling to fully relay to mesh friendship implementation
in the aspect of time management. Also, it adds different xmit settings
for the third (not in the friendship) device to avoid collisions.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>