[3.12] gh-109015: Add test.support.socket_helper.tcp_blackhole() (GH-109016) (#109041)

gh-109015: Add test.support.socket_helper.tcp_blackhole() (GH-109016)

Skip test_asyncio, test_imaplib and test_socket tests if FreeBSD TCP
blackhole is enabled (net.inet.tcp.blackhole=2).
(cherry picked from commit a52a3509770f29f940cda9307704908949912276)

Co-authored-by: Victor Stinner <vstinner@python.org>