Tests: blacklist beforeunload test in iOS

iOS has the window.onbeforeunload field but doesn't support the beforeunload
handler making it impossible to feature-detect the support.

I'd love to get a proper feature detect here but it would require to actually trigger the unload event and canceling it in `beforeunload` handler won't help in a browser that doesn't fire the event. :(

Why they exposed the field is beyond me.