test(16.2-06): keep happy-dom iframe pages offline in the editor tests

The preview tests put payloads with remote URLs into the preview frame's
srcdoc; happy-dom tried to load them (a DNS lookup of example.test on every
run). Tests must not reach the network.

vite.config.ts

@@ -31,5 +31,11 @@ export default defineConfig({
   test: {
     environment: 'happy-dom',
     include: ['tests/**/*.test.ts'],
+    // Tests never reach the network: happy-dom must not load the pages of
+    // iframes (the preview frame's srcdoc holds test payloads with remote
+    // URLs).
+    environmentOptions: {
+      happyDOM: { settings: { disableIframePageLoading: true } },
+    },
   },
 })