Go tests needing two local IPs: 127.0.0.2 fails on macOS, fall back to a LAN IPv4

A multi-node Go test that tells nodes apart by host (all nodes share one port) used 127.0.0.1 and 127.0.0.2. That works on Linux, where lo covers all of 127.0.0.0/8. On macOS, lo0 only has 127.0.0.1, so binding fails with 'bind: can't assign requested address' unless you run sudo ifconfig lo0 alias 127.0.0.2 up. Test-only fix: try 127.0.0.2 first; if it can't be bound, take the first non-loopback, non-link-local IPv4 from net.InterfaceAddrs() that can be bound; if there is none, t.Skip. Checked on macOS: a UDP socket bound to 127.0.0.1 and one bound to the LAN IP can send to each other in both directions. Mixing 127.0.0.1 with ::1 does not work, because an AF_INET socket cannot send to an IPv6 address. Caveat: binding a TCP listener on the LAN IP may trigger the macOS application firewall prompt on stricter setups.