tests-netsocket-udp: UDPSOCKET_ECHOTEST_NONBLOCK restores non-blocking

Fixes a bug where after dropping bad packets receiver doesn't restore
the socket back to non-blocking state as it's supposed to.

Author: Veijo Pesonen

TESTS/netsocket/udp/udpsocket_echotest.cpp

@@ -118,7 +118,7 @@ void udpsocket_echotest_nonblock_receiver(void *receive_bytes)
         }
     }
 
-    drop_bad_packets(sock, -1); // timeout equivalent to set_blocking(false)
+    drop_bad_packets(sock, 0); // timeout equivalent to set_blocking(false)
 
     tx_sem.release();
 }