Add tests for UnixDatagram from_raw_fd/into_raw_fd

tests/uds.rs

@@ -22,3 +22,21 @@ fn send_recv() -> io::Result<()> {
         Ok(())
     })
 }
+
+#[test]
+fn into_raw_fd() -> io::Result<()> {
+    use async_std::os::unix::io::{FromRawFd, IntoRawFd};
+    task::block_on(async {
+        let (socket1, socket2) = UnixDatagram::pair().unwrap();
+        socket1.send(JULIUS_CAESAR).await?;
+
+        let mut buf = vec![0; 1024];
+
+        let socket2 = unsafe { UnixDatagram::from_raw_fd(socket2.into_raw_fd()) };
+        let n = socket2.recv(&mut buf).await?;
+        assert_eq!(&buf[..n], JULIUS_CAESAR);
+
+        Ok(())
+    })
+
+}