Inclusion pruning tweaks (#1550)

In follow-up to https://github.com/paritytech/polkadot-sdk/pull/1518



Adding extra tests for inclusion pruning. Primarily focusing on various
cases surrounding candidates included in different forks (with different
relay parents).

All cases fall into a few buckets based on 3 degrees of freedom - number
of candidates, number of blocks (height), number of forks + extra case
for pruning multiple heights at once.

Added small tweak to the original pruning function to disregard stale
candidate duplicates which should keep the same behaviour.

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Co-authored-by: Tsvetomir Dimitrov <[email protected]>