Variable `isDev` is not used in both tests.
The tests run dev/prod modes based on the different test scenarios.
## Bug
- [x] Integration tests clean up
## Documentation / Examples
- [x] Make sure the linting passes
From https://github.com/vercel/next.js/pull/20628, when the page is rendered server-side, `Router`'s `isReady` field needs to be initially set to `true`. However, when `_app` has custom `getInitialProps`, it seems that it is not the case, even though the page is rendered on the server.
This leads to a bug that `Router.isReady` is never set to `true`.
This pull request fixes the problem by fixing the initial calculation logic of `isReady` of `Router`.
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [x] Make sure the linting passes
