caddy-frontend/test: update to new snapshot API

Adjust `self._cleanup` to new signature from slapos.core!156

/reviewed-on !677
6 jobs for fix/collective.recipe.shelloutput-return-code
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master

04:24:46

failed SlapOS.SoftwareReleases.IntegrationTest-Master

failed SlapOS.SoftwareReleases.IntegrationTest-Master

25:29:40

failed SlapOS.SoftwareReleases.IntegrationTest-Master

08:07:21

failed SlapOS.SoftwareReleases.IntegrationTest-Master

10:15:55

failed SlapOS.SoftwareReleases.IntegrationTest-Master

05:03:30