[th/run-nm-test-libtool] tests: improve detection of libtool in "run-nm-test.sh"

The user can set "$NMTST_LIBTOOL" to empty or the path to libtool.

If unset, we want to detect it. However, previously we would always use "$SCRIPT_PATH/../libtool", even if that file doesn't exit. Improve that.

For example, when running on Ubuntu with the ".gitlab-ci/debian-install.sh" script, we don't have libtool. Entering such a container and running the "run-nm-test.sh" script will fail "$SCRIPT_PATH/../libtool" doesn't exit.