malloc: Enable assertions if UNIT_TEST is enabled

dlmalloc has some sanity checks it performs on free() which can help detect
memory corruption. However, they are only enabled if DEBUG is defined before
including common.h. Define DEBUG earlier if UNIT_TEST is enabled so that
assertions are enabled in sandbox.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>