Fix missing MK_TESTS != "no" SUBDIR += tests logic

Equivalent functionality was removed from my copy of bsd.subdir.mk
some weeks ago
tap.test.mk -> bsd.test.mk and TEST_SUBDIRS -> SUBDIR for pjdfstest
Merge branch 'atf' into isilon-atf

Conflicts:
	lib/libc/Makefile
	lib/libthr/Makefile
	lib/msun/Makefile