gh-95243: Mitigate the race condition in testSockName (GH-96173)

find_unused_port() has an inherent race condition, but we can't use
bind_port() as that uses .getsockname() which this test is exercising.

Try binding to unused ports a few times before failing.

Signed-off-by: Ross Burton <ross.burton@arm.com>
(cherry picked from commit df110126)

Co-authored-by: Ross Burton <ross.burton@arm.com>