tests/pbkdf2: fix flake8 issue E741: ambiguous variable name
tests/pbkdf2: fix flake8 issue E741: ambiguous variable name

E741: ambiguous variable name

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