Fix tests for test_blocking_connection_errors();

A test function is comparing strings returned from gai_strerror() after an unsuccessful
call to getaddrinfo(), the problem is that the strings returned from gai_strerror() are implementation defined according to POSIX. 

I thought about adding REDIS_ERR_NONAME and set that based on the return value
from getaddrinfo (EAI_NONAME in this case) but as it happens OpenBSD fails to return
a proper error code (It returns -5, which I'm hunting down now) and I'm unsure how that would
play with things in the wild.