test: fix test-net-* error code check for getaddrinfo(3)

getaddrinfo(3) may (should?) return EAI_AGAIN on DNS errors. Some systems apparently returns 'ENOTFOUND' so we check both.

This makes the tests work with musl libc which correctly return EAI_AGAIN.

For valid errors see: http://pubs.opengroup.org/onlinepubs/009619199/getad.htm#tagcjh_05_06_05