test: handle vue-lib fixture for vite 6

[bluwy]

Description

Vite 6 library mode css file name now derives from the package.json "name" by default instead of always being style.css. This PR handles this case to work in both vite 5 and 6 by setting the new build.lib.cssFileName property.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[pkg-pr-new]

Open in Stackblitz

pnpm add https://pkg.pr.new/@vitejs/plugin-vue@465

pnpm add https://pkg.pr.new/@vitejs/plugin-vue-jsx@465

commit: 475ac0a