test: Isolate read-only-source-hmr test (#86721)
This was flakey with Turbopack:
```
Error: TIMED OUT: /New page/
404
This page could not be found.
undefined
at check (/root/actions-runner/_work/next.js/next.js/test/lib/next-test-utils.ts:726:9)
at Object.<anonymous> (/root/actions-runner/_work/next.js/next.js/test/integration/read-only-source-hmr/test/index.test.ts:124:7)
```
But somehow Webpack doesn't automatically refresh the page here?
https://github.com/user-attachments/assets/b363dbff-60d1-4813-bb1c-e5a64f0312d3
