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).