test: stabilize after() deploy revalidation checks (#96428)
## Summary
[PR #96354](https://github.com/vercel/next.js/pull/96354) changed
`retry()` to enforce its duration as a wall-clock budget instead of
effectively treating it as a retry count. This exposed the test’s
previous two-second budget, which did not reliably cover the intentional
one-second `after()` delay plus deployment and ISR latency.
Separates the callback delay from the ISR settling timeout and waits for
timestamp reads to stabilize before establishing a baseline.
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