Can you interact with the “Certificate” such as right click on the OK button to see the HTML of the button? You may have to do a second time to see the HTML of the button specifically. (Or use the F12 key and choose, Open DevTools, and pick the top left icon). If you can interact with the certificate, then perhaps you can try and create an object of either OK or Cancel button (or both) and then move to the certificate and click, like:
// move focus to the certificate page
WebUI.switchToWindowTitle('Select a certificate')
WebUI.click(findTestObject('Certificate Page/button_Ok'))
WebUI.switchToDefaultContent()
If you cannot interact with the certificate, then how about using browser options to prevent the certificate from showing.