Nuxt 4 + Layers breaks Nuxt Test Utils

Environment

- Operating System: Linux
- Node Version:     v20.16.0
- Nuxt Version:     3.17.2
- CLI Version:      3.25.0
- Nitro Version:    2.11.11
- Package Manager:  npm@10.8.1
- Builder:          -
- User Config:      modules, devtools, compatibilityDate
- Runtime Modules:  @nuxt/test-utils/module@3.18.0
- Build Modules:    -

Reproduction

Github repo: https://github.com/wuiyang/nuxt-layer-test-bug
Stackblitz: https://stackblitz.com/~/github.com/wuiyang/nuxt-layer-test-bug

Describe the bug

When project root is using Nuxt 4 version (via config future.compatibilityVersion = 4), nuxt test utils would not work on layers within same project.

The unit test would run without console error log, but the test report would show there is an error:

TypeError: Cannot read properties of undefined (reading 'vueApp')

When project root is not using Nuxt 4 version, the nuxt test utils would work as expected.
See reproduction repo:

• Github: https://github.com/wuiyang/nuxt-layer-test-bug/tree/working-without-nuxt-4
• Stackblitz: https://stackblitz.com/~/github.com/wuiyang/nuxt-layer-test-bug/tree/working-without-nuxt-4?file=package.json

Additional context

No response

Logs

abc@efac4cbfa0a5:/..../nuxt-layer-test-bug$ npx vitest run --coverage
 Vitest  "environmentMatchGlobs" is deprecated. Use "workspace" to define different configurations instead.
 HTML  Report is generated
       You can run npx vite preview --outDir ../test-reports/unit-test to see the test results.

Start with the linked reproduction repository and its package.json, comparing the working-without-nuxt-4 branch with the configuration using future.compatibilityVersion = 4. Run npx vitest run --coverage and verify that tests for layers complete without the undefined vueApp error and produce a valid report.