Tests: Blacklist one focusin test in IE

The main part of the test was checking that focusin handling in an iframe works
and that's still checked. The test was also checking that it doesn't propagate
to the parent document, though, and, apparently, in IE it does. This one test
is now blacklisted in IE.

test/unit/event.js

@@ -2561,7 +2561,14 @@ testIframe(
 
  // Create a focusin handler on the parent; shouldn't affect the iframe's fate
  jQuery( "body" ).on( "focusin.iframeTest", function() {
- assert.ok( false, "fired a focusin event in the parent document" );
+
+ // Support: IE 9 - 11+
+ // IE does propagate the event to the parent document. In this test
+ // we mainly care about the inner element so we'll just skip this one
+ // assertion in IE.
+ if ( !document.documentMode ) {
+ assert.ok( false, "fired a focusin event in the parent document" );
+ }
  } );
 
  input.on( "focusin", function() {