ci(test): use puppeteer bundled Chrome for Testing instead of system Chrome

Chrome 137+ removed the --load-extension flag from branded Google
Chrome builds, so the extension never loads under the system Chrome
provided by mymindstorm/puppeteer-headful and navigation to
chrome-extension://.../view/test.html fails with ERR_BLOCKED_BY_CLIENT.

Let npm ci download puppeteer's Chrome for Testing (which keeps
--load-extension) and run the headful tests under xvfb-run instead.

Author: Hank076

.github/workflows/main.yml

@@ -52,10 +52,6 @@ jobs:
 
       - name: Install dependencies
         run: npm ci
-        env:
-          PUPPETEER_SKIP_CHROMIUM_DOWNLOAD: "true"
 
       - name: Test code
-        uses: mymindstorm/puppeteer-headful@8f745c770f7f4c0f9f332d7c43a775f90e53779a
-        with:
-          args: npm test
+        run: xvfb-run --auto-servernum -- npm test