add testcase of Eth_TargetToDifficulty/Eth_DifficultyToTarget.
add testcase of Eth_TargetToDifficulty/Eth_DifficultyToTarget.

btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool