sigsys test: correct count of delivered signals

When knob is zero, intent is that no SIGSYS signals are delivered.
Comparing zero to zero does not test much, we should compare the count
of delivered SIGSYSs to zero.

Reviewed by: dchagin, imp
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44077