rtld: add a test for rtld_set_var (with LIBRARY_PATH_FDS)

PR:	294054
Reviewed by:	kib
MFC after:	1 week

libexec/rtld-elf/tests/Makefile

@@ -7,6 +7,7 @@ SUBDIR_DEPEND_target=	libpythagoras
 
 ATF_TESTS_C=	ld_library_pathfds
 ATF_TESTS_C+=	ld_preload_fds
+ATF_TESTS_C+=	set_var_test
 
 .for t in ${ATF_TESTS_C}
 SRCS.$t=	$t.c common.c

libexec/rtld-elf/tests/set_var_test.c

@@ -0,0 +1,38 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2026 Alex S <iwtcex@gmail.com>
+ */
+
+#include <atf-c.h>
+#include <dlfcn.h>
+#include <fcntl.h>
+#include <link.h>
+#include <stdio.h>
+
+ATF_TC_WITHOUT_HEAD(set_var_library_path_fds);
+ATF_TC_BODY(set_var_library_path_fds, tc)
+{
+	void *handle;
+	char *pathfds;
+	int testdir;
+
+	handle = dlopen("libpythagoras.so.0", RTLD_LAZY);
+	ATF_REQUIRE(handle == NULL);
+
+	testdir = open(atf_tc_get_config_var(tc, "srcdir"),
+	    O_RDONLY | O_DIRECTORY);
+	ATF_REQUIRE(testdir >= 0);
+
+	ATF_REQUIRE(asprintf(&pathfds, "%d", testdir) > 0);
+	ATF_REQUIRE(rtld_set_var("LIBRARY_PATH_FDS", pathfds) == 0);
+
+	handle = dlopen("libpythagoras.so.0", RTLD_LAZY);
+	ATF_REQUIRE(handle != NULL);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, set_var_library_path_fds);
+	return atf_no_error();
+}