scripts: twister: add timeout for pytest process

Add protection timeout for pytest subprocess, to avoid situation of
suspending whole Twister in case of internal pytest test problem.

Co-authored-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>
Signed-off-by: Piotr Golyzniak <piotr.golyzniak@nordicsemi.no>