Add tap.test.mk.

This file provides support to build test programs that comply with the
Test Anything Protocol.  Its main goal is to support the painless
integration of existing tests from tools/regression/ into the Kyua-based
test suite.

Approved by:	rpaulo (mentor)
Make bsd.progs.mk work in directories with SCRIPTS but no PROGS.

This change fixes some cases where bsd.progs.mk would fail to handle
directories with SCRIPTS but no PROGS.  In particular, "install" did
not handle such scripts nor dependent files when bsd.subdir.mk was
added to the mix.

This is "make tinderbox" clean.

Reviewed by:	freebsd-testing
Approved by:	rpaulo (mentor)