Make "kerberos" test suite independent of "localhost" name resolution.

Make "kerberos" test suite independent of "localhost" name resolution.

This suite malfunctioned if the canonical name of "localhost" was
something other than "localhost", such as "localhost.localdomain".  Use
hostaddr=127.0.0.1 and a fictitious host=, so the resolver's answers for
"localhost" don't affect the outcome.  Back-patch to v11, which
introduced this test suite.

Discussion: https://postgr.es/m/20180801050903.GA1392916@rfd.leadboat.com