testing/snapper: add bash completion, use /etc/snapper in cron scripts

snapper is configured with --with-conf=/etc/snapper, hence whenever a user wants to create a new config (e.g. snapper -c root create-config /) it will try to inject config name to /etc/snapper/snapper. That file doesn't exist as we currently install it into /etc/conf.d. Meanwhile the cron files look for config names in /etc/conf.d/snapper.

I don't know what we should do here (either put config files in /etc/snapper or /etc/conf.d).