[gitlabci] Moved test prefix creation to before_script and deletion to after_script

This ensure that whatever the outcome of the tests, the prefix is
cleaned up. Currently these are piling up in the /var/tmp folder.
1 job from fix_backup_restore in 23 minutes 43 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #125904
docker-build
build

23:43

71.0%