[test] Deflake custom cache handler test (#85610)
[flakiness
metric](https://app.datadoghq.com/ci/test/runs?query=test_level%3Atest%20%40git.repository.id%3A%22github.com%2Fvercel%2Fnext.js%22%20%40test.name%3A%22use-cache-custom-handler%20should%20use%20a%20modern%20custom%20cache%20handler%20if%20provided%22%20%40test.type%3A%22nextjs%22%20%40test.status%3A%22fail%22%20%40git.branch%3Acanary&agg_m=count&agg_m_source=base&agg_t=count&fromUser=true&index=citest&start=1759310750664&end=1761902750664&paused=false)
The test page used a low revalidate time of 3 seconds. In CI, sometimes
that much time passes between the first and second request, causing the
test to fail when comparing the data values (locally reproducible with
`{ cpuThrottleRate: 20 }`). Increasing the revalidate time to 6 seconds
should make the test more robust.
