malloc: Turn on DEBUG when enabling unit tests

Unit tests require mallinfo which in turn requires DEBUG on
dlmalloc to be enabled.

The dependancy on CONFIG_SANDBOX is wrong.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>