test: Check block number against parent number when validating block

Author: gumb0

test/blockchaintest/blockchaintest_runner.cpp

@@ -116,6 +116,9 @@ bool validate_block(
     if (parent_header == nullptr)
         return false;
 
+    if (test_block.block_info.number != parent_header->block_number + 1)
+        return false;
+
     if (test_block.block_info.gas_used > test_block.block_info.gas_limit)
         return false;