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 988d99ad)

Fixes #14717
Closes gh-1496

test/unit/event.js

@@ -1400,7 +1400,10 @@ test("Submit event can be stopped (#11049)", function() {
 });
 
 // Test beforeunload event only if it supported (i.e. not Opera)
-if ( window.onbeforeunload === null ) {
+// Support: iOS 7+
+// iOS has the window.onbeforeunload field but doesn't support the beforeunload
+// handler making it impossible to feature-detect the support.
+if ( window.onbeforeunload === null && !/(ipad|iphone|ipod)/i.test( navigator.userAgent ) ) {
 	asyncTest("on(beforeunload)", 4, function() {
 		var win,
 			forIE6 = 0,