add pov-recovery unit tests and support for elastic scaling (#4733)

- unit tests for pov-recovery
- elastic scaling support (recovering multiple candidates in a single
relay chain block)
- also some small cleanups
- also switches to candidates_pending_availability in
`handle_empty_block_announce_data`

Fixes https://github.com/paritytech/polkadot-sdk/issues/3577

After https://github.com/paritytech/polkadot-sdk/pull/4097 is merged, we
should also add a zombienet test, similar to the existing
`0002-pov_recovery.toml` but which has a single collator using elastic
scaling on multiple cores.