test-acl-util: output more debug info

For some reason this failed in koji build on s390x:
--- command ---
16:12:46 PATH='/builddir/build/BUILD/systemd-stable-246.1/s390x-redhat-linux-gnu:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/sbin' SYSTEMD_LANGUAGE_FALLBACK_MAP='/builddir/build/BUILD/systemd-stable-246.1/src/locale/language-fallback-map' SYSTEMD_KBD_MODEL_MAP='/builddir/build/BUILD/systemd-stable-246.1/src/locale/kbd-model-map' /builddir/build/BUILD/systemd-stable-246.1/s390x-redhat-linux-gnu/test-acl-util
--- stdout ---
-rw-r-----. 1 mockbuild mock 0 Aug  7 16:12 /tmp/test-empty.7RzmEc
other::---
--- stderr ---
Assertion 'r >= 0' failed at src/test/test-acl-util.c:42, function test_add_acls_for_user(). Aborting.