Ensure that the test postmaster started by 'make check' listens to as

Ensure that the test postmaster started by 'make check' listens to as
few 'listen_addresses' as possible --- on most systems, none at all,
just the Unix socket.  This avoids spurious check failures due to bogus
DNS setups, and is probably a good idea from a security standpoint anyway.
Per trouble report from Jean-GÅrard Pailloncy.