Fix k5test.py hostname canonicalization

Fix k5test.py hostname canonicalization

r25844 (#7124) stopped using AI_ADDRCONFIG when canonicalizing
hostnames in sn2princ.  So we need to also stop using it in k5test.c's
_get_hostname() or we could come up with a different result on a
system where forward and reverse resolution via IPv4 and IPv6 produce
different results.  That in turn causes a t_gssapi.py test (the one
using the un-canonicalized hostname) to fail, because libkrb5 looks
for a different host principal than k5test.py put in the keytab.