This bypasses any issues we might have with containers where IPv6 is
returned by name resolution (particularly since !616) but
doesn't necessarily actually work.

This comes at a minor test-coverage cost: we don't test GDBusServer's
default behaviour when told to listen on "tcp:" or "nonce-tcp:", and
on systems where IPv6 is available, we don't test it. If we want to
do those, we should perhaps do them in separate tests, and disable
those tests when binding to ::1 doesn't work.

Mitigates: #1912


Signed-off-by: Simon McVittie <smcv@collabora.com>