BOSTON — Zachary Sevigny was slashed with a box cutter by a stranger outside a convenience store in 2011. Neither Sevigny nor his friend identified Jeremy Gomes as the attacker when shown his picture in a police photo array. But a week later, they saw Gomes inside a Pittsfield gas station and told police he was the culprit. Gomes was found guilty of the attack, but his lawyer is challenging his conviction based on what he says were unreliable eyewitness identifications. The case is one of four cases seeking …