Split and extend bsd.test.mk into {atf,plain,tap}.test.mk.

Split and extend bsd.test.mk into {atf,plain,tap}.test.mk.

This is a MFC of:

- r256761 Clearly split the logic to build ATF and plain tests apart.
- r256762 Add the automatic generation of Atffile files.
- r256763 Add the automatic generation of Kyuafile files.
- r256764 Plug atf-run into the 'test' target.
- r256765 Plug kyua into the 'test' target.
- r257096 Move the TESTSBASE definition to bsd.own.mk.
- r257099 Add missing plain.test.mk.
- r258297 Remove registration of C++ test programs into PROGS.
- r258298 Fix the build of plain test programs.
- r258551 Install plain.test.mk.
- r259208 Add tap.test.mk.

Approved by:	rpaulo (mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/10@259962 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f