When snap1 doesn't exist, `lxc-snapshot` will create a snapshot named
`snap1` (instead of `snap2`). In this case we need to rename snap1 into
snap2.
However the condition check is wrong: after creating the snapshot,
`snap1` will *always* exist – even if it didn't before. Thus the
renaming never happens.
Fix the issue by detecting the absence of snap2 (rather than snap1).
Usage de snapshot pour les installations de base.
Réduit la durée des tests en évitant de réinstaller l'application plusieurs fois avec les mêmes paramètres.