Fix improper use of MK_TESTS here from r264996.

In stable/10, the MK_TESTS=no form is not supported.  This was preventing
the bmake bootstrap from working.

Sponsored by:	EMC / Isilon Storage Division

Makefile

@@ -358,7 +358,7 @@ MMAKE=		${MMAKEENV} ${MAKE} \
 		-D_UPGRADING \
 		-DNOMAN -DNO_MAN -DNOSHARED -DNO_SHARED \
 		-DNO_CPU_CFLAGS -DNO_WERROR \
-		DESTDIR= MK_TESTS=no PROGNAME=${MYMAKE:T}
+		DESTDIR= -DNO_TESTS PROGNAME=${MYMAKE:T}
 
 make bmake: .PHONY
 	@echo