Meta: Use `QT_QPA_PLATFORM` offscreen in `WPT.sh`

This has better compatibility with MacOS.

Meta/WPT.sh

@@ -121,7 +121,7 @@ execute_wpt() {
             fi
             WPT_ARGS+=( "--webdriver-arg=--certificate=${certificate_path}" )
         done
-        QT_QPA_PLATFORM="minimal" ./wpt run "${WPT_ARGS[@]}" ladybird "${TEST_LIST[@]}"
+        QT_QPA_PLATFORM="offscreen" ./wpt run "${WPT_ARGS[@]}" ladybird "${TEST_LIST[@]}"
     popd > /dev/null
 }