tests/aip31068: Mark internal functions as static
tests/aip31068: Mark internal functions as static

Also prefix them with an underscore to prevent naming conflicts

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