Build and install the subr_unit test program originally written by phk, and
run it with the other ATF tests.
tests/sys/kern/Makefile
* Build and install the subr_unit test as a plain test
sys/kern/subr_unit.c
* Reduce the default number of repetitions from 100 to 1, and add a
command-line parser to override it.
* Don't be so noisy by default
* Fix an include problem for the test build
Reviewed by: ngie
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6038
794277da54