test: speed up UT by external @rsbuild/core

[9aoy]

Summary

test: speed up UT by external @rsbuild/core.

@rsbuild/core has already been built, so there is no need to build it again in test.

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[Copilot]

Pull Request Overview

This PR externalizes the @rsbuild/core package in the test runner configuration to avoid rebuilding it during unit tests and improve test performance.

• Adds an output section with externals pointing to @rsbuild/core in rstest.config.ts

[netlify]

✅ Deploy Preview for rsbuild ready!

Name	Link
🔨 Latest commit	f7722b5
🔍 Latest deploy log	https://app.netlify.com/projects/rsbuild/deploys/686c89be0c0a3e0007d94c93
😎 Deploy Preview	https://deploy-preview-5558--rsbuild.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 74 (🔴 down 3 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: 60 (no change from production) View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[chenjiahan]

• before: pnpm ut 13.69s user 3.06s system 623% cpu 2.685 total
• after: pnpm ut 10.67s user 2.36s system 816% cpu 1.595 total