test: add libkvm read test

This test checks if value received from kvm_read is sane, based on
value returned by sysctl interface.

This should catch regression on bug fixed by r359160

Reviewed by: jhb
Approved by: jhibbits (mentor)
MFC after: 1 week
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D23783