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

@@ -68,6 +68,10 @@ test_fdw_auth($node1, $db0, "t2", $fdw_server2,
 test_auth($node2, $db2, "t2",
 	"SCRAM auth directly on foreign server should still succeed");
 
+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');
@@ -108,6 +112,7 @@ like(
 		$stderr,
 		qr/failed: authentication method requirement "scram-sha-256"/,
 		'expected error from loopback password (different cluster)');
+}
 
 # Helper functions