std: Draw from the same port pool during tests

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

Author: alexcrichton

src/libstd/net/test.rs

@@ -14,14 +14,14 @@ use env;
 use net::{SocketAddr, IpAddr};
 use sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
 
+static PORT: AtomicUsize = ATOMIC_USIZE_INIT;
+
 pub fn next_test_ip4() -> SocketAddr {
-    static PORT: AtomicUsize = ATOMIC_USIZE_INIT;
     SocketAddr::new(IpAddr::new_v4(127, 0, 0, 1),
                     PORT.fetch_add(1, Ordering::SeqCst) as u16 + base_port())
 }
 
 pub fn next_test_ip6() -> SocketAddr {
-    static PORT: AtomicUsize = ATOMIC_USIZE_INIT;
     SocketAddr::new(IpAddr::new_v6(0, 0, 0, 0, 0, 0, 0, 1),
                     PORT.fetch_add(1, Ordering::SeqCst) as u16 + base_port())
 }