MFC r363020: test: add libkvm read test

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

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