Add missing const in mod_aux.c

Signed-off-by: ChangZhuo Chen (陳昌倬) <czchen@gmail.com>
Refactoring
Add missing const in testhelper.h

Signed-off-by: ChangZhuo Chen (陳昌倬) <czchen@gmail.com>
Add const in mod_aux APIs

Signed-off-by: ChangZhuo Chen (陳昌倬) <czchen@gmail.com>