Migrate route-exit.js to Vitest (part of #882)

[umer2001m]

This PR migrates the route-exit.js test file from Mocha to Vitest as part of the v6 release tracking issue (#882).

✅ Rewrites the test file using Vitest syntax
✅ Updates package.json test script to use Vitest

This contributes to Yeoman's upcoming v6 release and aligns with modern test tooling practices.

resolves #882.