MFC r320441,r320442:

MFC r320441,r320442:

r320441:

share/examples/tests/Makefile: clean up example snippets/documentation

- TESTSDIR doesn't need to be specified after r289158.
- Including bsd.own.mk isn't required since no MK_<foo> knobs are being
  manipulated.
- TESTS_SUBDIRS should be written out in an append format, one entry
  per line, to provide a better, more conflict resistant example.

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.