This is a simple proof of concept test, that gnrc_sock with gnrc_neterr
works as expected to report errors within the network stack up to the sock
object.
It just tests if the sock reports -EHOSTUNREACH when sending to an unreachable
address. To confirm that this is indeed coming from the stack and not the sock
wrapper you can do
git grep EHOSTUNREACH -- sys/net/gnrc/sock