Prevent building tests when bootstrapping make.

Should fix the breakage reported by tinderbox when WITHOUT_BMAKE is set,
which was probably introduced in r263346..

Makefile

@@ -330,7 +330,8 @@ MMAKEENV=	MAKEOBJDIRPREFIX=${MYMAKE:H} \
 		INSTALL="sh ${.CURDIR}/tools/install.sh"
 MMAKE=		${MMAKEENV} ${MAKE} \
 		-D_UPGRADING -DNO_MAN -DNO_SHARED \
-		-DNO_CPU_CFLAGS -DNO_WERROR DESTDIR= PROGNAME=${MYMAKE:T}
+		-DNO_CPU_CFLAGS -DNO_WERROR \
+		DESTDIR= MK_TESTS=no PROGNAME=${MYMAKE:T}
 
 make bmake: .PHONY
 	@echo