utils_test: check stderr to catch sanitizer issues

Recent OS#3407 shows that we should verify stderr to catch sanitizer failures.
(They might not always be ignorable like that one.)

Change-Id: Ic9e437a1cc96ae081e0fd6a9b6e3156987e14c0c
diff --git a/tests/Makefile.am b/tests/Makefile.am
index db4e520..eaaa8df 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -240,7 +240,7 @@
 	     vty/ok_tabs_and_spaces.cfg \
 	     vty/ok_tabs.cfg \
 	     comp128/comp128_test.ok bits/bitfield_test.ok		\
-	     utils/utils_test.ok stats/stats_test.ok			\
+	     utils/utils_test.ok utils/utils_test.err stats/stats_test.ok \
 	     bitvec/bitvec_test.ok msgb/msgb_test.ok bits/bitcomp_test.ok \
 	     sim/sim_test.ok tlv/tlv_test.ok abis/abis_test.ok		\
 	     gsup/gsup_test.ok gsup/gsup_test.err			\