This change introduces a new plain.test.mk file that provides the build
infrastructure to build test programs that don't use any framework.
Most of the code previously in bsd.test.mk moves to plain.test.mk and
atf.test.mk is extended with the missing pieces.
In doing so, this change pushes all test program building logic to the
various *.test.mk files instead of trying to reuse some tiny bits.
In fact, this attempt to reuse some definitions makes the code harder
to read and harder to extend.
The clear benefit of this is that the interface of bsd.test.mk is now
clearly delimited.
Submitted by: Julio Merino jmmv google.com
MFC after: 2 weeks
d4a14c8563