Automate the subr_unit test.

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