Simplify running the FreeBSD test suite

Replace make regress (legacy test make target) and make test (incomplete
test make target added with the FreeBSD test suite) with make check as it's
consistent with other open source projects.

make check defaults to running tests from .OBJDIR, but can be overridden
with the CHECKDIR variable.

Add make checkworld target to simplify running the FreeBSD test suite from
TESTSBASE (i.e. the top-level tests directory), similar to buildworld.

Document make check and make checkworld in build(7).

Other minor changes:

• Rename intermediate file (Kyuafile.auto) to Kyuafile to simplify make check.
• Remove terse warnings attached to beforetest/aftertest.
• Add kyua binary check to check target in suite.test.mk; error out if it's not found

The MFC is [partly] contingent on other build related changes being MFCed.

Differential Revision: https://reviews.freebsd.org/D4406
MFC after: 2 months
X-MFC to: stable/10
Relnotes: yes
Reviewed by: bdrewery, Evan Cramer <eccramer@gmail.com>
Sponsored by: EMC / Isilon Storage Division