Skip TAP test for peer authentication if there are no unix-domain sockets

Peer connections require support for local connections to work, but the
test missed the same check as the other ones in this suite.  The
buildfarm does not run the authentication tests on Windows, and, more
surprisingly, the CI with meson was already able to skip it.

Author: Anton A. Melnikov
Discussion: https://postgr.es/m/28b9d685-9590-45b1-fe87-358d61c6950a@inbox.ru

Author: michaelpq

src/test/authentication/t/003_peer.pl

@@ -2,13 +2,19 @@
 # Copyright (c) 2021-2022, PostgreSQL Global Development Group
 
 # Tests for peer authentication and user name map.
-# The test is skipped if the platform does not support peer authentication.
+# The test is skipped if the platform does not support peer authentication,
+# and is only able to run with Unix-domain sockets.
 
 use strict;
 use warnings;
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 use Test::More;
+if (!$use_unix_sockets)
+{
+	plan skip_all =>
+	  "authentication tests cannot run without Unix-domain sockets";
+}
 
 # Delete pg_hba.conf from the given node, add a new entry to it
 # and then execute a reload to refresh it.