Ignore a test which never completes on windows

I'm not entirely sure why this is happening, but the server task is never seeing
the second send of the client task, and this test will very reliably fail to
complete on windows.

src/libstd/rt/uv/uvio.rs

@@ -2306,6 +2306,7 @@ fn test_read_read_read() {
 }
 
 #[test]
+#[ignore(cfg(windows))] // FIXME(#10102) the server never sees the second send
 fn test_udp_twice() {
     do run_in_mt_newsched_task {
         let server_addr = next_test_ip4();