chore: prevent accidental npm install in test fixtures

[tido64]

Description

Prevent accidental npm install in test fixtures

Test plan

% npm i
npm error code EBADENGINE
npm error engine Unsupported engine
npm error engine Not compatible with your version of node/npm: AwesomeGame@1.0.0
npm error notsup Not compatible with your version of node/npm: AwesomeGame@1.0.0
npm error notsup Required: {"node":"Do not install; for testing purposes only"}
npm error notsup Actual:   {"npm":"10.9.3","node":"v22.19.0"}

% pnpm i
 ERR_PNPM_UNSUPPORTED_ENGINE  Unsupported environment (bad pnpm and/or Node.js version)

Your Node version is incompatible with "/~/incubator/polyfills/test/__fixtures__".

Expected version: Do not install; for testing purposes only
Got: v22.19.0

This is happening because the package's manifest has an engines.node field specified.
To fix this issue, install the required Node version.

% yarn
Usage Error: The project in /~/package.json doesn't seem to have been installed - running an install there might help

Yarn configs are inherited and will always fail because test fixtures are not part of workspaces.