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.