GH-79033: Fix asyncio.Server.wait_closed() (#98582)

It was a no-op when used as recommended (after close()).

I had to debug one test (test__sock_sendfile_native_failure) --
the cleanup sequence for the test fixture was botched.

Hopefully that's not a portend of problems in user code --
this has never worked so people may well be doing this wrong. :-(

Co-authored-by: kumar aditya