Fix TcpStream::connect_timeout on linux

Linux appears to set POLLOUT when a conection's refused, which is pretty
weird. Invert the check to look for an error explicitly. Also add an
explict test for this case.

Closes #45265.

src/libstd/net/tcp.rs

@@ -1579,6 +1579,21 @@ mod tests {
                 "bad error: {} {:?}", e, e.kind());
     }
 
+    #[test]
+    fn connect_timeout_unbound() {
+        // bind and drop a socket to track down a "probably unassigned" port
+        let socket = TcpListener::bind("127.0.0.1:0").unwrap();
+        let addr = socket.local_addr().unwrap();
+        drop(socket);
+
+        let timeout = Duration::from_secs(1);
+        let e = TcpStream::connect_timeout(&addr, timeout).unwrap_err();
+        assert!(e.kind() == io::ErrorKind::ConnectionRefused ||
+                e.kind() == io::ErrorKind::TimedOut ||
+                e.kind() == io::ErrorKind::Other,
+                "bad error: {} {:?}", e, e.kind());
+    }
+
     #[test]
     fn connect_timeout_valid() {
         let listener = TcpListener::bind("127.0.0.1:0").unwrap();