Clean up all directories created by make hier

The logic I introduced in r322511 unfortunately left chflags schg'ed
directories behind created by `make hier` (in the stock /etc/mtree
files, this is limited to /var/empty).

The proposed change calls `chflags -R 0` and `rm -Rf ...` to clean all
of the directories that could not be removed by `${MAKE} clean`.
`${MAKE} clean` in bsd.obj.mk calls `cleandir`/`cleanobj`, which handles
the first directory tree walk/removal.

Approved by:	emaste (mentor)
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D18641

share/mk/suite.test.mk

@@ -120,5 +120,7 @@ beforecheck:
 #       etc.
 aftercheck:
 	@cd ${.CURDIR} && ${MAKE} clean
+	@chflags -R 0 "${DESTDIR}"
+	@rm -Rf "${DESTDIR}"
 
 .endif