blockchaintest: Only check state root for 5 first/last blocks

This is blockchain tests execution optimization: only check state root
hash of first 5 blocks (to detect early problems) and last 5 blocks
(to do the final check of the chain of blocks).

The current implementation of the MPT hash of the state builds
the trie from scratch (no updates to the trie of the previous block).
For the tests will a long chain of blocks the performance degrades
significantly with 99% time spent in the keccak hash function.

This improves testing of EIP-2935
implemented in #953.

test/blockchaintest/blockchaintest_runner.cpp

@@ -141,8 +141,9 @@ void run_blockchain_tests(std::span<const BlockchainTest> tests, evmc::VM& vm)
         std::unordered_map<int64_t, hash256> known_block_hashes{
             {c.genesis_block_header.block_number, c.genesis_block_header.hash}};
 
-        for (const auto& test_block : c.test_blocks)
+        for (size_t i = 0; i != c.test_blocks.size(); ++i)
         {
+            const auto& test_block = c.test_blocks[i];
             auto bi = test_block.block_info;
             bi.known_block_hashes = known_block_hashes;
 
@@ -157,8 +158,17 @@ void run_blockchain_tests(std::span<const BlockchainTest> tests, evmc::VM& vm)
             SCOPED_TRACE(std::string{evmc::to_string(rev)} + '/' + std::to_string(case_index) +
                          '/' + c.name + '/' + std::to_string(test_block.block_info.number));
 
-            EXPECT_EQ(
-                state::mpt_hash(TestState{state}), test_block.expected_block_header.state_root);
+            static constexpr size_t STATE_HASH_CHECK_MARGIN = 5;
+            if (i < STATE_HASH_CHECK_MARGIN || i >= c.test_blocks.size() - STATE_HASH_CHECK_MARGIN)
+            {
+                // Only check state hashes for first (early problems)
+                // and last (final check of the chain) blocks.
+                // Otherwise, for very long blockchain test we spend a lot of time in MPT hashing
+                // because the implementation of it is very simplistic: the trie is not updated
+                // along the state changes but a new trie is build from scratch for every block.
+                EXPECT_EQ(
+                    state::mpt_hash(TestState{state}), test_block.expected_block_header.state_root);
+            }
 
             if (rev >= EVMC_SHANGHAI)
             {