selftests: af_unix: don't use SKIP for expected failures

netdev CI reserves SKIP in selftests for cases which can't be executed
due to setup issues, like missing or old commands. Tests which are
expected to fail must use XFAIL.

Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/af_unix/unix_connreset.c

···
 	char buf[16] = {};
 	ssize_t n;
 
-	if (variant->socket_type == SOCK_DGRAM)
-		SKIP(return, "This test only applies to SOCK_STREAM and SOCK_SEQPACKET");
+	if (variant->socket_type == SOCK_DGRAM) {
+		snprintf(_metadata->results->reason,
+			 sizeof(_metadata->results->reason),
+			 "Test only applies to SOCK_STREAM and SOCK_SEQPACKET");
+		exit(KSFT_XFAIL);
+	}
 
 	/* Close server without accept()ing */
 	close(self->server);