Backport tests from a jQuery 3.x fix that's not needed on `master`.
Also, fix the "focusin from an iframe" test to actually verify the behavior
from commit 1cecf64e5a - the commit that
introduced the regression - to make sure we don't regress on either front.
The main part of the modified 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.
(cherry picked from 9e15d6b469)
(cherry picked from 1a4f10ddc3)
Ref gh-4652
Ref gh-4656
Closes gh-4657
9b73204350