test: Fix immutable static asset deployment tests (#95550)
1. The env var ended up having a different name than I foresaw months
ago. Now `next.assetToken` is correctly `undefined` if the deployment
used immutable static assets.
2. `expect(searchParams.get(...)).toBe(undefined)` was failing because
`.get()` returns null for unknown keys
Failures:
-
https://github.com/vercel/next.js/actions/runs/28831699621/job/85515363446
-
https://github.com/vercel/next.js/actions/runs/28831699621/job/85515363310
