[Testing] Finish tests for "Fix crash closing Popup with WebView"

[PureWeen]

Description

#21718 currently only happens when using 3rd party library mopups. The fixes that @jsuarezruiz put in come down to better checking if a handler has been disconnected after the navigation has completed.

We're still working on reproducing this without pulling in mopups, my guess is that we just need to time a call to "DisconnectHandler" right when navigation has started.