This test sometimes times out when the machine is busy.
The reason behind is still a bit blurry. But it seems to comes from
the fact that on AIX, once a listen is performed a socket, every
connection will be accepted even before an accept is made (which only
occurs when a machine is busy). On Linux, a socket is created as a
"passive socket" which seems to wait for the accept before allowing
incoming connections.

Updates #29685

Change-Id: I41b053b7d5f5b4420b72d6a217be72e41220d769
Reviewed-on: https://go-review.googlesource.com/c/go/+/185717
Run-TryBot: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>