Alternator is most likely actually bad despite the good test. Do you remember what voltage it generated? The reason the vehicle died during driving is that the battery was not charging from the alternator. The vehicle eventually depleted the battery's charge and had no power left to keep itself running. In addition, the battery you purchased may not have been fully charged, or the vehicle discharged it on the way home leaving the computer systems to use the rest of the charge over the course of the week. A larger electrical issue is always a possibility, but a bad alternator is the most probable and a good starting point. Good luck!