Unflake parallel-routes-revalidation test (#76600)

[flakiness
metric](https://app.datadoghq.com/ci/test/runs?query=test_level%3Atest%20env%3Aci%20%40git.repository.id%3Agithub.com%2Fvercel%2Fnext.js%20%40test.service%3Anextjs%20%40test.status%3Afail%20%28-%40git.branch%3A%2A%3F%2A%20OR%20%40git.branch%3Acanary%29%20%40test.name%3A%22parallel-routes-revalidation%20should%20not%20trigger%20the%20intercepted%20route%20when%20lazy-fetching%20missing%20data%22&agg_m=count&agg_m_source=base&agg_t=count&citest_explorer_sort=timestamp%2Casc&cols=%40test.status%2Ctimestamp%2C%40test.suite%3A83%2C%40test.name%3A607%2C%40git.branch&currentTab=overview&eventStack=&fromUser=false&graphType=flamegraph&index=citest&start=1738068572273&end=1740660572273&paused=false)

The latest canary release
[failed](https://github.com/vercel/next.js/actions/runs/13555525831/job/37890389323)
because the deploy test `parallel-routes-revalidation › should not
trigger the intercepted route when lazy-fetching missing data` kept
failing even with retries.

According to my local testing using `pnpm test-deploy
test/e2e/app-dir/parallel-routes-revalidation/parallel-routes-revalidation.test.ts
-t 'should not trigger the intercepted route when lazy-fetching missing
data'`, the forward navigation was sometimes triggered too early after
the page was reloaded.

To fix it we're waiting for the idle network event before proceeding
with the forward navigation.

Author: unstubbable

Diff for: test/e2e/app-dir/parallel-routes-revalidation/parallel-routes-revalidation.test.ts

@@ -176,6 +176,7 @@ describe('parallel-routes-revalidation', () => {
 
     // reload the page, which will cause the router to no longer have cache nodes
     await browser.refresh()
+    await browser.waitForIdleNetwork()
 
     // go forward, this will trigger a lazy fetch for the missing data, and should restore the detail page
     await browser.forward()