test: cover prerendered remote functions during adapter SSR

[teemingc]

This is test coverage only, not a fix for #15035. The assertions currently verify rendered output, not whether the response was read locally instead of fetched over HTTP or recomputed.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from bbfcd1b:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/bbfcd1b86c625a72a8965b384f3c9092f996f5de

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17366

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-17366-svelte.vercel.app/