test: Adjust difficulty value if below minimal difficulty (0x20000)

Author: rodiazet

test/state/ethash_difficulty.cpp

@@ -32,6 +32,8 @@ int64_t get_bomb_delay(evmc_revision rev) noexcept
 int64_t calculate_difficulty_pre_byzantium(int64_t parent_difficulty, int64_t parent_timestamp,
     int64_t current_timestamp, int64_t block_number, evmc_revision rev)
 {
+    static constexpr auto MIN_DIFFICULTY = int64_t{0x20000};
+
     // According to https://eips.ethereum.org/EIPS/eip-2
     const auto period_count = block_number / 100'000;
     const auto offset = parent_difficulty / 2048;
@@ -48,7 +50,8 @@ int64_t calculate_difficulty_pre_byzantium(int64_t parent_difficulty, int64_t pa
     else if (period_count == 2)
         diff += 1;
 
-    return diff;
+    // Adjust if below min difficulty `0x20000`
+    return std::max(MIN_DIFFICULTY, diff);
 }
 
 }  // namespace