Auto merge of #22857 - alexcrichton:net-flaky, r=alexcrichton

Auto merge of #22857 - alexcrichton:net-flaky, r=alexcrichton

Instead of allocating the same ports for ipv4 and ipv6 tests, instead draw all
ports from the same pool. Some tests connect to just "localhost" on a particular
port which may accidentally be interacting with other tests as the ipv-what-ness
isn't specified with the string "localhost"

Relevant logs:

* [Deadlock of the `net::tcp::tests::listen_localhost` test][mac]
* [Failure of the `fast_rebind` test][win1]
* [Failure of `multiple_connect_interleaved_lazy_schedule_ip4`][win2]

[mac]: https://gist.github.com/alexcrichton/349c7ce7c620c1adb2f2
[win1]: https://gist.github.com/alexcrichton/7e3611faae2e1edaee6f
[win2]: https://gist.github.com/alexcrichton/4f5f87749af3ad0f9851