sandbox_spl: Increase SPL malloc() size

The test framework reinits driver model tests before running each test.
Since malloc_simple does not support free(), this eventually runs out of
memory.

Fix it for now by increasing the space to 32KB.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>