test: assumeutxo snapshot 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 the snapshot block was
submitted after loading the snapshot and downloading a few blocks after the
snapshot. In that case ReceivedBlockTransactions() previously would overwrite
the nChainTx value of the submitted snapshot block with a fake value based on
the previous block, so the (pindex->nChainTx == pindex->nTx + prev_chain_tx)
check would later fail on the first block after the snapshot. This test was
originally posted by Martin Zumsande <[email protected]> in
bitcoin/bitcoin#29370 (comment)

Co-authored-by: Martin Zumsande <[email protected]>

Author: ryanofsky

test/functional/feature_assumeutxo.py

@@ -324,6 +324,16 @@ def check_tx_counts(final: bool) -> None:
 
         self.log.info("Restarted node before snapshot validation completed, reloading...")
         self.restart_node(1, extra_args=self.extra_args[1])
+
+        # Send snapshot block to n1 out of order. This makes the test less
+        # realistic because normally the snapshot block is one of the last
+        # blocks downloaded, but its useful to test because it triggers more
+        # corner cases in ReceivedBlockTransactions() and CheckBlockIndex()
+        # setting and testing nChainTx values, and it exposed previous bugs.
+        snapshot_hash = n0.getblockhash(SNAPSHOT_BASE_HEIGHT)
+        snapshot_block = n0.getblock(snapshot_hash, 0)
+        n1.submitblock(snapshot_block)
+
         self.connect_nodes(0, 1)
 
         self.log.info(f"Ensuring snapshot chain syncs to tip. ({FINAL_HEIGHT})")