PATCH: add ATF tests in sys

[Alan Somers]

I rewrote the unix seqpacket tests in ATF.  The hard part is adding
them to the build.  The linked patch adds them in the directory
sys/kern/tests and builds them as part of buildworld, not buildkernel.
 They get installed to /usr/tests/sys/kern, but the intermediate
objects are stored in /usr/obj/sys/kern/tests.  That means that you
can't have different tests associated with different kernel configs.
I think that this is desirable, because you wouldn't be able to
install tests for different kernel configs anyway, given our chosen
layout of /usr/tests.

Please comment on the parts of this patch that deal with Makefiles.
Is this the appropriate way to add sys tests to the build?  Shouldn't
I be building them in buildkernel instead of buildworld?  I couldn't
find a good way to do that.

https://gist.github.com/asomers/5f98656d574e68314029