test: Fail cgroup delegation test when user cannot be created

It means: a) user cannot be created, something's wrong in the
test environment -> fail the test; b) user already exists, we shall not
continue and delete (foreign) user.

test/units/TEST-19-CGROUP.delegate.sh

@@ -74,7 +74,7 @@ testcase_attributes() {
 
 testcase_scope_unpriv_delegation() {
     # Check that unprivileged delegation works for scopes
-    useradd test ||:
+    useradd test
     trap "userdel -r test" RETURN
     systemd-run --uid=test \
                 --property="User=test" \