Move `collectSpanNamesUntilSegment` helper into `@sentry-internal/test-utils`

[chargome]

The helper is duplicated across nextjs-app-dir, nextjs-16, nextjs-16-bun and nextjs-16-cf-workers — it should live in dev-packages/test-utils instead.

Raised in review of #23905.

[linear-code]

JS-3537