MFC Note: ${FILES} documentation change omitted since r299094 will
not be MFCed to ^/stable/10 .

MFC r320442:

share/examples/tests/{atf,plain}/Makefile: tweak example Makefile snippets

- Including bsd.own.mk isn't required since no MK_<foo> knobs are being
manipulated.
- Update documentation to note that ${FILES} is installed via bsd.progs.mk,
not bsd.prog.mk.

MFC refactoring of the *.test.mk files.

- r263161 Make bsd.test.mk the only public mk fragment for the building of tests.
- r263172 Move FreeBSD Test Suite-specific code to a suite.test.mk file.
- r263204 Add some documentation for bsd.test.mk.
- r263217 Document support for TAP-compliant Perl test programs.

This is "make tinderbox" clean.

MFC refactoring of the *.test.mk files.

- r263161 Make bsd.test.mk the only public mk fragment for the building of tests.
- r263172 Move FreeBSD Test Suite-specific code to a suite.test.mk file.
- r263204 Add some documentation for bsd.test.mk.
- r263217 Document support for TAP-compliant Perl test programs.

This is "make tinderbox" clean.