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.

MFC after:      1 month

share/examples/tests/tests/Makefile

@@ -1,7 +1,5 @@
 # $FreeBSD$
 
-.include <bsd.own.mk>
-
 # Directory into which the Kyuafile provided by this directory will be
 # installed.
 #
@@ -11,12 +9,16 @@
 #
 # For example: if this Makefile were in src/bin/cp/tests/, its TESTSDIR
 # would point at ${TESTSBASE}/bin/cp/.
-TESTSDIR=	${TESTSBASE}/share/examples/tests
+#
+# The default path specified by bsd.test.mk is `${TESTSBASE}/${RELDIR:H}`,
+# which happens to be the same as `${TESTSBASE}/share/examples/tests`.
+#TESTSDIR=	${TESTSBASE}/share/examples/tests
 
 # List of subdirectories into which we want to recurse during the build
 # of the system.  We use TESTS_SUBDIRS instead of SUBDIR because we want
 # the auto-generated Kyuafile to recurse into these directories.
-TESTS_SUBDIRS=	atf plain
+TESTS_SUBDIRS+=	atf
+TESTS_SUBDIRS+=	plain
 
 # We leave KYUAFILE unset so that bsd.test.mk auto-generates a Kyuafile
 # for us based on the contents of the TESTS_SUBDIRS line above.  The