Draft: dh_update_autotools_config: Backup and restore Autotest testsuite scripts

Projects using Autotest automatically regenerate the testsuite script before running it.

See: https://www.gnu.org/software/autoconf/manual/autoconf-2.71/html_node/Making-testsuite-Scripts.html

To stop dpkg-source complaining about such files being modified during in-place rebuilds, let's backup the original scripts and restore them during dh_clean.