twister: Use natural sort when generating hardware map

Use the natural sort of list when generating a hardware map. The
list is sorted with a serial port as a key. When more than 10 ports
are active and some of devices use more than one port, the ports
of one device may be listed in wrong sequence, which may cause
futher problems with selecting the right port for listening to
the device.

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