Fix: Use ipv6 loopback adress in test_uri

Some older version of Debian/Ubuntu and maybe other distros won't set
localhost to point on ::1 in the system hosts file and use instead a
custom name like ip6-localhost or ipv6-locahost which is not
standardized across distros.

To work around this, use the ipv6 loopback address instead of localhost.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>