Make tests build on CloudABI.

There are some tests that need to be disabled on CloudABI specifically,
due to the fact that the shims cannot be built in combination with
unix::ext or windows::ext. Also improve the scoping of some imports to
suppress compiler warnings.

src/libstd/net/udp.rs

@@ -786,7 +786,7 @@ impl fmt::Debug for UdpSocket {
     }
 }
 
-#[cfg(all(test, not(target_os = "emscripten")))]
+#[cfg(all(test, not(any(target_os = "cloudabi", target_os = "emscripten"))))]
 mod tests {
     use io::ErrorKind;
     use net::*;