tests/pbkdf2: increase stack for AVR
tests/pbkdf2: increase stack for AVR

ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT