ci: fix test runner

Author: JakobVogelsang

web-test-runner.config.mjs

@@ -2,6 +2,8 @@
 import { visualRegressionPlugin } from '@web/test-runner-visual-regression/plugin';
 import { playwrightLauncher } from '@web/test-runner-playwright';
 
+import { polyfill } from '@web/dev-server-polyfill';
+
 import pixelmatch from 'pixelmatch';
 import { PNG } from 'pngjs';
 
@@ -58,6 +60,9 @@ function defaultGetImageDiff({ baselineImage, image, options }) {
 
 export default /** @type {import("@web/test-runner").TestRunnerConfig} */ ({
   plugins: [
+    polyfill({
+      scopedCustomElementRegistry: true,
+    }),
     visualRegressionPlugin({
       update: process.argv.includes('--update-visual-baseline'),
       getImageDiff: (options) => {