twister: Ensure serial-pty process termination

Make sure Twster DeviceHandler serial-pty process is terminated
with all its remaining children to avoid Twister hanging on it
infinitely.
The reolved issue occurs sometimes, for example when serial-pty
script is used for serial port tunneling over network.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>