test: assumeutxo stale block CheckBlockIndex crash test

Add a test for a CheckBlockIndex crash that would happen before previous
"assumeutxo: Get rid of faked nTx and nChainTx values" commit.

The crash was an assert failure in the (pindex->nChainTx == pindex->nTx +
prev_chain_tx) check that would previously happen if a snapshot was loaded, and
a block was submitted which forked from the chain before the snapshot block and
after the last downloaded background chain block. This block would not be
marked assumed-valid because it would not be an ancestor of the snapshot, and
it would have nTx set, nChainTx unset, and prev->nChainTx set with a fake
value, so the assert would fail. After the fix, prev->nChainTx is unset instead
of being set to a fake value, so the assert succeeds. This test was originally
posted by maflcko in
bitcoin#29261 (comment)

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>

test/functional/feature_assumeutxo.py

@@ -173,6 +173,14 @@ def run_test(self):
  hash = n0.getbestblockhash()
  newblock = n0.getblock(hash, 0)
  blocks[height] = Block(hash, block_tx, blocks[height-1].chain_tx + block_tx)
+ if i == 4:
+ # Create a stale block that forks off the main chain before the snapshot.
+ temp_invalid = n0.getbestblockhash()
+ n0.invalidateblock(temp_invalid)
+ stale_hash = self.generateblock(n0, output="raw(aaaa)", transactions=[], sync_fun=self.no_op)["hash"]
+ n0.invalidateblock(stale_hash)
+ n0.reconsiderblock(temp_invalid)
+ stale_block = n0.getblock(stale_hash, 0)
 
  # make n1 aware of the new header, but don't give it the block.
  n1.submitheader(newblock)
@@ -246,6 +254,15 @@ def check_tx_counts(final: bool) -> None:
 
  assert_equal(n1.getblockchaininfo()["blocks"], SNAPSHOT_BASE_HEIGHT)
 
+ self.log.info("Submit a stale block that forked off the chain before the snapshot")
+ # Normally a block like this would not be downloaded, but if it is
+ # submitted early before the background chain catches up to the fork
+ # point, it winds up in m_blocks_unlinked and triggers a corner case
+ # that previously crashed CheckBlockIndex.
+ n1.submitblock(stale_block)
+ n1.getchaintips()
+ n1.getblock(stale_hash)
+
  self.log.info("Submit a spending transaction for a snapshot chainstate coin to the mempool")
  # spend the coinbase output of the first block that is not available on node1
  spend_coin_blockhash = n1.getblockhash(START_HEIGHT + 1)