test(rsc): fix ssr modulepreload link test

[hi-ogawa]

Description

__vite_rsc_assets_manifest.js shouldn't be import-ed during the test since the module will be cached inside playwright worker but the content can change between different test variants (e.g. build-default and build-cloudflare). This was making rolldown run to fail on first run (then retry always succeeds since playwright recreate a worker) https://github.com/vitejs/vite-plugin-react/actions/runs/16235033401/job/45843973297#step:9:681

I already did this for basic.test.ts

vite-plugin-react/packages/plugin-rsc/e2e/basic.test.ts

Lines 236 to 241 in b2b9276

	const manifest = JSON.parse(
	readFileSync(
	f.root + '/dist/ssr/__vite_rsc_assets_manifest.js',
	'utf-8',
	).slice('export default '.length),
	)

, but react-router.test.ts still had the old one.