oauth: Use IPv4-only issuer in oauth_validator tests

oauth: Use IPv4-only issuer in oauth_validator tests

The test authorization server implemented in oauth_server.py does not
listen on IPv6. Most of the time, libcurl happily falls back to IPv4
after failing its initial connection, but on NetBSD, something is
consistently showing up on the unreserved IPv6 port and causing a test
failure.

Rather than deal with dual-stack details across all test platforms,
change the issuer to enforce the use of IPv4 only. (This elicits more
punishing timeout behavior from libcurl, so it's a useful change from
the testing perspective as well.)

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Reported-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: http://postgr.es/m/CAOYmi%2Bn4EDOOUL27_OqYT2-F2rS6S%2B3mK-ppWb2Ec92UEoUbYA%40mail.gmail.com