Allow unit tests to be written in packages/next/src (#45222)

Allows `.test.ts` to be colocated next to files so that unit tests can be written in the Next.js core codebase.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/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`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

jest.config.js

@@ -9,6 +9,7 @@ const customJestConfig = {
   setupFilesAfterEnv: ['<rootDir>/jest-setup-after-env.ts'],
   verbose: true,
   rootDir: 'test',
+  roots: ['<rootDir>', '<rootDir>/../packages/next/src/'],
   modulePaths: ['<rootDir>/lib'],
   transformIgnorePatterns: ['/next[/\\\\]dist/', '/\\.next/'],
   globals: {