postgres_fdw: Fix tests on some Windows variants

The tests introduced by commit 76563f88cf only work when Unix-domain
sockets are available.  This is optional on Windows, and buildfarm
member drongo runs without them.  To fix, skip the test if Unix-domain
sockets are not enabled.

contrib/postgres_fdw/t/001_auth_scram.pl

@@ -68,46 +68,51 @@ test_fdw_auth($node1, $db0, "t2", $fdw_server2,
 test_auth($node2, $db2, "t2",
 	"SCRAM auth directly on foreign server should still succeed");
 
-# Ensure that trust connections fail without superuser opt-in.
-unlink($node1->data_dir . '/pg_hba.conf');
-unlink($node2->data_dir . '/pg_hba.conf');
+SKIP:
+{
+	skip "test requires Unix-domain sockets", 4 if !$use_unix_sockets;
+
+	# Ensure that trust connections fail without superuser opt-in.
+	unlink($node1->data_dir . '/pg_hba.conf');
+	unlink($node2->data_dir . '/pg_hba.conf');
 
-$node1->append_conf(
-	'pg_hba.conf', qq{
+	$node1->append_conf(
+		'pg_hba.conf', qq{
 local   db0             all                                     scram-sha-256
 local   db1             all                                     trust
 }
-);
-$node2->append_conf(
-	'pg_hba.conf', qq{
+	);
+	$node2->append_conf(
+		'pg_hba.conf', qq{
 local   all             all                                     password
 }
-);
-
-$node1->restart;
-$node2->restart;
-
-my ($ret, $stdout, $stderr) = $node1->psql(
-	$db0,
-	qq'select count(1) from t',
-	connstr => $node1->connstr($db0) . " user=$user");
-
-is($ret, 3, 'loopback trust fails on the same cluster');
-like(
-	$stderr,
-	qr/failed: authentication method requirement "scram-sha-256"/,
-	'expected error from loopback trust (same cluster)');
-
-($ret, $stdout, $stderr) = $node1->psql(
-	$db0,
-	qq'select count(1) from t2',
-	connstr => $node1->connstr($db0) . " user=$user");
-
-is($ret, 3, 'loopback password fails on a different cluster');
-like(
-	$stderr,
-	qr/failed: authentication method requirement "scram-sha-256"/,
-	'expected error from loopback password (different cluster)');
+	);
+
+	$node1->restart;
+	$node2->restart;
+
+	my ($ret, $stdout, $stderr) = $node1->psql(
+		$db0,
+		qq'select count(1) from t',
+		connstr => $node1->connstr($db0) . " user=$user");
+
+	is($ret, 3, 'loopback trust fails on the same cluster');
+	like(
+		$stderr,
+		qr/failed: authentication method requirement "scram-sha-256"/,
+		'expected error from loopback trust (same cluster)');
+
+	($ret, $stdout, $stderr) = $node1->psql(
+		$db0,
+		qq'select count(1) from t2',
+		connstr => $node1->connstr($db0) . " user=$user");
+
+	is($ret, 3, 'loopback password fails on a different cluster');
+	like(
+		$stderr,
+		qr/failed: authentication method requirement "scram-sha-256"/,
+		'expected error from loopback password (different cluster)');
+}
 
 # Helper functions