wpt: use a pool of browser to run tests

.github/workflows/wpt.yml

@@ -108,7 +108,7 @@ jobs:
         run: |
           ./wpt serve 2> /dev/null & echo $! > WPT.pid
           sleep 10s
-          ./wptrunner -lpd-path ./lightpanda -json -concurrency 6 > wpt.json
+          ./wptrunner -lpd-path ./lightpanda -json -concurrency 10 -pool 3 > wpt.json
           kill `cat WPT.pid`
 
       - name: write commit