Products.TIDStorage was not ported to python3 and is not installed on
software-py3.cfg but the backup crontab expects tidstorage to be
present - as a result, it was silently failing to produce backups.

This brings minimal support to repozo backups on python3, without
Products.TIDStorage interraction and also extends software release test
to have a simple test checking that backups are produced and can be
restored.