Add test to demonstrate the failure scenario

[rahulksnv]

The change adds a test to show the failure scenario that caused #1812 to be rolled back (more context: #493 (comment))

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)

[dmitry-markin]

Is it possible to modify the test so that it's testing not the failure scenario but the expected behavior? E.g., if we remove [should_panic] and the test passes, that means that we fixed the bug?

[rahulksnv]

Is it possible to modify the test so that it's testing not the failure scenario but the expected behavior? E.g., if we remove [should_panic] and the test passes, that means that we fixed the bug?

yes, if the issue is resolved, two changes would be needed:

1. [should_panic] removed
2. Minor change to the last part would be needed. The last part should result in no requests being sent out, test should check for that instead

[rahulksnv]

@dmitry-markin Addressed comments, please merge if looks good, thanks

[dmitry-markin]

@dmitry-markin Addressed comments, please merge if looks good, thanks

Looks good to me, but still need one more approving review to be able to merge.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable-int
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4140199

[dmitry-markin]

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