The `doc` variable in:
https://github.com/jquery/jquery/blob/3.4.1/src/event/focusin.js#L30
matched `document` for `document` & `window` for `window`, creating two
separate wrapper event handlers & calling handlers twice if at least one
`focusout` or `focusin` handler was attached on *both* `window` & `document`,
or on `window` & another regular node.

Also, fix the "focusin from an iframe" test to actually verify the behavior
from commit 1cecf64 - the commit that
introduced the regression - to make sure we don't regress on either front.

Fixes gh-4652
Closes gh-4656

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 1cecf64 - 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 9e15d6b)
(cherry picked from 1a4f10d)

Ref jquerygh-4652
Ref jquerygh-4656

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 1cecf64 - 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 9e15d6b)
(cherry picked from 1a4f10d)

Ref gh-4652
Ref gh-4656
Closes gh-4657