caddy-frontend/test: update to new snapshot API

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

/reviewed-on nexedi/slapos!677
1 job for feature/promise-cleanup in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/promise-cleanup

11:12:16