[tests] fix ip generation boundaries

https://ci.kronosnet.org/job/knet-build-all-voting/1450/knet-build-all-voting=rhel80z-s390x/console

and similar, when pid = 255, the secondary IP would hit 256 that is of course invalid.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>