chore: remove splitVendorChunkPlugin from tests

[sapphi-red]

Description

fixes this ecosystem-ci failure: https://github.com/vitejs/vite-ecosystem-ci/actions/runs/15061007152/job/42336016840#step:8:901

Additional context

---

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

npm i https://pkg.pr.new/@vitejs/plugin-vue@589

npm i https://pkg.pr.new/@vitejs/plugin-vue-jsx@589

commit: 5aa033c