fix UDP test

Test may fail because localhost might be resolved as '::1' (IPv6) whereas server listens on IPv4 only (0.0.0.0).

This might be also fixed by using 'localhost' as addres for both server and client or use IPv6 (::) for server which will posibly (platform dependent) go dual stack. But the second option expects IPv6 urned on by default, which I try to avoid.