RIOT/tests/gnrc_sock_neterr

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