test: refactor some style tests

[Timeless0911]

Summary

• remove root to avoid warning logs of failed to load package.json
• no need to set lightningcssLoader: false when using postcss
• resolve Sass variables aren't allowed in plain CSS issue in tests
• use postcss.config.cjs instead of postcss.config.ts to avoid warning in higher Node.js version
• await the assertion to remove unexpected feature which will be removed in Vitest 3.0

Related Links

Checklist

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

[SoonIter]

thanks❤