Define a new token to simplify checking whether fence_malloc() actually
can catch out-of-bounds access.

This will be used in the future to skip tests that rely on fence_malloc
checking functionality.

Changes in v2:

- #define FENCE_MALLOC_ACTIVE always, but change its value to help catch
  use of it without including utils.h

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ben Avison <bavison@riscosopen.org>