tests(udp): use blocking socket to receive empty datagram (#2339)

The `handle_empty_datagram` unit tests sends and receives an empty datagram. To
prevent a race condition where the receiver tries to receive the datagram before
it is available (error `WouldBlock`), use a blocking socket via `socket()`
instead.

neqo-udp/src/lib.rs

@@ -233,7 +233,7 @@ mod tests {
         // platforms. Use `std` socket instead.  See also
         // <https://github.com/quinn-rs/quinn/pull/2123>.
         let sender = std::net::UdpSocket::bind("127.0.0.1:0")?;
-        let receiver = Socket::new(std::net::UdpSocket::bind("127.0.0.1:0")?)?;
+        let receiver = socket()?;
         let receiver_addr: SocketAddr = "127.0.0.1:0".parse().unwrap();
 
         sender.send_to(&[], receiver.inner.local_addr()?)?;