Introduce BlockchainTests without mining

[pirapira]

#318 (comment)

yes. so lets say if blockchaintest.json has "sealEngine" : "NoProof" then difficulty should not be calculated.
if .json has "sealEngine" : "Ethash" then it works as usual.
if .json has no "sealEngine" field then it should fail as the test is outdated

[winsvega]

so @holiman said that having such a blockchain tests will require other clients to disable consensus critical part of the code. and Hive is designed to be a black box tester providing tests as if it was on a real network.
it was agreed to optimize cpp blockchain tests generation instead and keep blockchain tests mining on the lowest allowed difficulty.