unr(9) iterator: add naive test

To use, compile userspace code e.g. into the subr_unit binary, then do
$ while ./subr_unit -iv >|/tmp/subr_unit.log ; do :; done
The loop should be left run for as long as possible.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D40089