Since 27a1b2b ("chore(core): Flatten FST store code"), the bucket loop in
`FstStorePool::dump_action` checked the type and name of the collection
entry instead of the bucket entry. As a collection is a directory, every
bucket was skipped: `TRIGGER backup` answered `OK` but wrote no FST, and
`TRIGGER restore` restored none.
Add a test which backs up a consolidated FST and restores it.