The change adds a test to show the failure scenario that caused #1812 to
be rolled back (more context:
https://github.com/paritytech/polkadot-sdk/issues/493#issuecomment-1772009924)

Summary of the scenario:
1. Node has finished downloading up to block 1000 from the peers, from
the canonical chain.
2. Peers are undergoing re-org around this time. One of the peers has
switched to a non-canonical chain, announces block 1001 from that chain
3. Node downloads 1001 from the peer, and tries to import which would
fail (as we don't have the parent block 1000 from the other chain)

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Co-authored-by: Dmitry Markin <[email protected]>