savecore: add a test for LOG_PERROR

Signed-off-by: Stéphane Rochoy <stephane.rochoy@stormshield.eu>

Reviewed by:	markj, oshogbo
MFC after:	2 weeks
Pull Request:	https://github.com/freebsd/freebsd-src/pull/1546

sbin/savecore/tests/Makefile

@@ -1,3 +1,3 @@
-ATF_TESTS_SH=	livedump_test
+ATF_TESTS_SH=	livedump_test log_test
 
 .include <bsd.test.mk>

sbin/savecore/tests/log_test.sh

@@ -0,0 +1,25 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2025 Stéphane Rochoy <stephane.rochoy@stormshield.eu>
+#
+
+atf_test_case log_perror
+log_perror_head()
+{
+	atf_set "descr" "Test LOG_PERROR behavior"
+}
+log_perror_body()
+{
+	atf_check -s exit:1 \
+	          -o ignore \
+	          -e save:savecore.err \
+	    savecore -vC /dev/missing
+	grep -qE 'savecore [0-9]+ - - /dev/missing: No such file or directory' savecore.err \
+	    || atf_fail "missing/invalid error output"
+}
+
+atf_init_test_cases()
+{
+	atf_add_test_case log_perror
+}