Use MK_CHECK_USE_SANDBOX in tests/..., to deal with the fact that

tests/... is a special snowflake directory and using HAS_TESTS would
result in a nasty layering violation between bsd.tests.mk and
bsd.prog.mk.

Add reachover Makefile.inc's which get the default value from
Makefile.inc0 (inspired by gnu/usr.bin/binutils/Makefile.inc0).

tests/Makefile

@@ -17,4 +17,5 @@ afterinstall: install-tests-local
 install-tests-local: .PHONY
 	${INSTALL_SYMLINK} ../local/tests ${DESTDIR}${TESTSDIR}/local
 
+.include "Makefile.inc0"
 .include <bsd.test.mk>

tests/Makefile.inc0

@@ -0,0 +1,9 @@
+# $FreeBSD$
+
+# tests/... doesn't conform to bsd.lib.mk and bsd.prog.mk, so specify a sane
+# default for MK_CHECK_USE_SANDBOX. src.opts.mk will override the value if the
+# user sets it to no.
+MK_CHECK_USE_SANDBOX=	yes
+
+.include <src.opts.mk>
+

tests/etc/Makefile.inc

@@ -0,0 +1,3 @@
+# $FreeBSD$
+
+.include "${SRCTOP}/tests/Makefile.inc0"

tests/sys/Makefile.inc

@@ -1,3 +1,5 @@
 # $FreeBSD$
 
 WARNS?=		6
+
+.include "${SRCTOP}/tests/Makefile.inc0"

tests/sys/geom/class/Makefile.inc

@@ -0,0 +1,3 @@
+# $FreeBSD$
+
+.include "${SRCTOP}/tests/Makefile.inc0"

tests/sys/pjdfstest/Makefile.inc

@@ -0,0 +1,3 @@
+# $FreeBSD$
+
+.include "${SRCTOP}/tests/Makefile.inc0"

tests/sys/pjdfstest/tests/Makefile.inc

@@ -0,0 +1,3 @@
+# $FreeBSD$
+
+.include "../Makefile.inc"