test: adjust assertions for /_not-found being possibly added to prerender manifest#3043

pieh wants to merge 1 commit intomainopennextjs/opennextjs-netlify:mainfrom

test-not-found-in-prerender-manifest-canaryopennextjs/opennextjs-netlify:test-not-found-in-prerender-manifest-canaryCopy head branch name to clipboard