test: attempt to de-flake rsc-basic (#77934)
I've been seeing this test flake with some regularity:
---
● app dir - rsc basics › should be able to navigate between rsc routes
```
request.allHeaders: Target page, context or browser has been closed
168 | page.on('request', (request) => {
169 | requestsCount++
> 170 | return request.allHeaders().then((headers) => {
| ^
171 | if (
172 | headers['RSC'.toLowerCase()] === '1' &&
173 | // Prefetches also include `RSC`
at Page.allHeaders (e2e/app-dir/rsc-basic/rsc-basic.test.ts:170:26)
```
---
[Example test run
here](https://github.com/vercel/next.js/actions/runs/14313872710/job/40115445718?pr=77898#step:33:3431)
The error is actually unrelated to `should be able to navigate between
rsc routes`, and is coming from async operations (that haven't completed
in time) in another test in the same suite, `should reuse the inline
flight response without sending extra requests` .
I've de-asyncified the test's `page.on('request')` handler -- we don't
need the async `allHeaders()`, `headers()` is enough.
(I verified this by adding a client component that manually sends a
request with `RSC: 1`, and that does indeed trigger our request
listener).
I've also added a `waitForIdleNetwork` to hopefully make it complete all
requests before the test ends.
