[WebUI] Verify Alert Present

This is a companion discussion topic for the original entry at https://docs.katalon.com/katalon-studio/docs/webui-verify-alert-present.html
'Verify alert is present after 20 seconds'

The verify and wait should be in xx seconds, not after.
The alert maybe show up in xx seconds, the system should check whether the alert show up repeatly.
otherwise, after xx seconds, the alert maybe disapeared after showing up.