scripts: twister: Add support for passing extra serial port to pytest

Extend twister's device serial handling to support multiple serial ports
for pytest-based tests. This enables testing scenarios that require
communication with multiple UART interfaces on the same device.
This enhancement enables comprehensive testing of multi-UART devices
while maintaining backward compatibility with single serial setups.

Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>