cumulus: bump PARENT_SEARCH_DEPTH and add test for 12-core elastic scaling (#6983)

On top of https://github.com/paritytech/polkadot-sdk/pull/6757

Fixes https://github.com/paritytech/polkadot-sdk/issues/6858 by bumping
the `PARENT_SEARCH_DEPTH` constant to a larger value (30) and adds a
zombienet-sdk test that exercises the 12-core scenario.

This is a node-side limit that restricts the number of allowed pending
availability candidates when choosing the parent parablock during
authoring.
This limit is rather redundant, as the parachain runtime already
restricts the unincluded segment length to the configured value in the
[FixedVelocityConsensusHook](https://github.com/paritytech/polkadot-sdk/blob/88d900af

/cumulus/pallets/aura-ext/src/consensus_hook.rs#L35)
(which ideally should be equal to this `PARENT_SEARCH_DEPTH`).

For 12 cores, a value of 24 should be enough, but I bumped it to 30 to
have some extra buffer.

There are two other potential ways of fixing this:
- remove this constant altogether, as the parachain runtime already
makes those guarantees. Chose not to do this, as it can't hurt to have
an extra safeguard
- set this value to be equal to the uninlcuded segment size. This value
however is not exposed to the node-side and would require a new runtime
API, which seems overkill for a redundant check.

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Co-authored-by: Javier Viola <javier@parity.io>