Use 'WITHOUT_TESTS=1' instead of 'MK_TESTS=no' in the

'distribute' target of etc/Makefile, because we do not
allow command-line use of 'make MK_TESTS=no' in stable/10.

This fixes a regression introduced in r270187 that causes
the release build to fail, and a direct commit to stable/10.

Sponsored by:	The FreeBSD Foundation

etc/Makefile

@@ -176,7 +176,7 @@ afterinstall:
 distribute:
 	# Avoid installing tests here; "make distribution" will do this and
 	# correctly place them in the right location.
-	${_+_}cd ${.CURDIR} ; ${MAKE} MK_TESTS=no install \
+	${_+_}cd ${.CURDIR} ; ${MAKE} WITHOUT_TESTS=1 install \
 	    DESTDIR=${DISTDIR}/${DISTRIBUTION}
 	${_+_}cd ${.CURDIR} ; ${MAKE} distribution DESTDIR=${DISTDIR}/${DISTRIBUTION}