Tests: Blacklist beforeunload test in iOS
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.

(cherry-picked from 988d99ad278c2c9dc16c68f86bddc4df99dcd928)

Fixes #14717
Closes gh-1496

diracdeltas/jquery,diracdeltas/jquery,sancao2/jquery,sancao2/jquery,diracdeltas/jquery,sancao2/jquery