p_shadow-utils: use next guid for groupadd test

There is a possibility that the guid 1010 is in used during the test and
could make it fail. Picking the next available guid works around that
possibility, while letting the test still test the same scenario.

tests/p_shadow-utils/30-groupadd_tests

@@ -7,5 +7,6 @@ t_Log "Running $0 group add tests"
 sed -i /testgroup/d /etc/group
 
 echo "Create a new group"
-groupadd -g 1010 testgroup
+next_guid=$((10+$(awk -F: '{print $3}' /etc/group | sort | tail -1)))
+groupadd -g $next_guid testgroup
 t_CheckExitStatus $?