libc/sys: add errno test

Despite looking trivial, it requires proper split of exports from libsys
and libc, proper filtering work in rtld, and operational libsys.

Reviewed by:	emaste, imp
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D44075

lib/libc/tests/sys/Makefile

@@ -7,6 +7,7 @@ PACKAGE=			tests
 ATF_TESTS_C+=			brk_test
 .endif
 ATF_TESTS_C+=			cpuset_test
+ATF_TESTS_C+=			errno_test
 ATF_TESTS_C+=			queue_test
 ATF_TESTS_C+=			sendfile_test
 

lib/libc/tests/sys/errno_test.c

@@ -0,0 +1,36 @@
+/*-
+ * Copyright (c) 2024 The FreeBSD Foundation
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * This software were developed by Konstantin Belousov <kib@FreeBSD.org>
+ * under sponsorship from the FreeBSD Foundation.
+ */
+
+#include <errno.h>
+#include <unistd.h>
+
+#include <atf-c.h>
+
+ATF_TC(errno_basic);
+ATF_TC_HEAD(errno_basic, tc)
+{
+	atf_tc_set_md_var(tc, "descr",
+	    "Verify basic functionality of errno");
+}
+
+ATF_TC_BODY(errno_basic, tc)
+{
+	int res;
+
+	res = unlink("/non/existent/file");
+	ATF_REQUIRE(res == -1);
+	ATF_REQUIRE(errno == ENOENT);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, errno_basic);
+
+	return (atf_no_error());
+}