QubesOS/qubes-issues#9402

The former is removed in Python 3.13

QubesOS/qubes-issues#9402

Don't leave children, as cleanup_loop will (rightfully) detect it as a
leak.

Don't leak child processes after the test, even if they would terminate
after a timeout anyway.

Include content of the selector map when it's non-empty, to ease finding
what got leaked.

Recent Thunderbird opens a message in a window no fitting on 1024x768 by
default. This makes the attachment button off-screen so clicking it
doesn't work. Fix it by resizing the window.

Python 3.12+ fixed server.wait_closed() to really wait for all the
connections to terminate. This causes qubesd to hang on shutdown, as
events connections were not closed. Python 3.13 introduces convenient
server.close_clients() function to close all connections.

There was a similar code in tests already - convert that to
close_clients() call too.

Fixes QubesOS/qubes-issues#9427

Python 3.13 adds automatic socket cleanup - specifically, a
cleanup_socket parameter to loop.create_unix_server, and it defaults to
True. Manual cleanup is not needed anymore.