test: fix missing new future flag

Author: edmundhung

integration/vite-presets-test.ts

@@ -246,6 +246,7 @@ test.describe("Vite / presets", async () => {
         unstable_optimizeDeps: true,
         unstable_subResourceIntegrity: false,
         unstable_trailingSlashAwareDataRequests: false,
+        unstable_previewServerPrerendering: false,
         v8_middleware: true,
         v8_splitRouteModules: false,
         v8_viteEnvironmentApi: false,