test: ensure pnpm type support test works when @eslint/core is updated

[fasttime]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Prevent an error in the pnpm type support test of config-helpers when @eslint/core is updated to a version that hasn't been published to npm yet. This happens when a Release Please PR is updated or merged. Examples in CI:

• https://github.com/eslint/rewrite/actions/runs/20809981279/job/59771734925
• https://github.com/eslint/rewrite/actions/runs/20856061902/job/59922903851

What changes did you make? (Give an overview)

Added a pnpm-worspace.yaml file in the test directory to override the location of @eslint/core. With this configuration, pnpm no longer attempts to install @eslint/core in node_modules, but node_modules/.pnpm is still populated with the files pnpm needs to resolve it. I verified that the pnpm type support test still fails es expected if tools/dedupe-types.js is reverted to remove the changes introduced in #289.

Related Issues

Is there anything you'd like reviewers to focus on?

[eslint-github-bot]

Hi @fasttime!, thanks for the Pull Request

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[lumirlumir]

LGTM, thanks!