tests/periph_adc: fix content printf on failure
tests/periph_adc: fix content printf on failure

mtausig/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,josephnoir/RIOT,kYc0o/RIOT,mtausig/RIOT,toonst/RIOT,OlegHahm/RIOT,josephnoir/RIOT,miri64/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,x3ro/RIOT,x3ro/RIOT,aeneby/RIOT,toonst/RIOT,aeneby/RIOT,josephnoir/RIOT,x3ro/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,authmillenon/RIOT,yogo1212/RIOT,miri64/RIOT,smlng/RIOT,miri64/RIOT,OTAkeys/RIOT,basilfx/RIOT,aeneby/RIOT,basilfx/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,aeneby/RIOT,smlng/RIOT,toonst/RIOT,yogo1212/RIOT,authmillenon/RIOT,aeneby/RIOT,mtausig/RIOT,yogo1212/RIOT,mtausig/RIOT,yogo1212/RIOT,ant9000/RIOT,OTAkeys/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,OlegHahm/RIOT,miri64/RIOT,x3ro/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,smlng/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,smlng/RIOT,x3ro/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,toonst/RIOT,smlng/RIOT,yogo1212/RIOT,OTAkeys/RIOT,josephnoir/RIOT,mtausig/RIOT,basilfx/RIOT,toonst/RIOT