nethtest: blockhash doesn't match other clients, part II

[holiman]

New version of #3201

What @MariusVanDerWijden in #3201 was almost true, but somewhat simplified.

Example case 565138.json which was submitted by @guidovranken
It has currentNumber set:

    "env": {
      "currentGasLimit": "0xb71b00",
      "currentTimestamp": "0x03e8",
      "previousHash": "0x5e20a0453cecd065ea59c37ac63e079ee08998b6045136a8ce6635c7912ec0b6",
      "currentCoinbase": "0x2adc25665018aa1fe0e6bc666dac8fc2697ff9ba",
      "currentDifficulty": "0x020000",
      "currentBaseFee": "0x0a",
      "currentRandom": "0x0000000000000000000000000000000000000000000000000000000000020000",
      "currentNumber": "0xc5d488"

What @MariusVanDerWijden wrote is only true if we execute on BLOCKNUMBER 1 , in which case 0 is the only one "eligible" for blockhash. If we're on a later number, there are other blocks which are 'eligible'.
Some printouts from a run:

{"pc":27,"op":64,"gas":"0x8910","gasCost":"0x14","memSize":32,"stack":["0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b","0x0","0x0","0x20","0x18","0x0"],"depth":97,"refund":0,"opName":"BLOCKHASH"}
Requested blockhash 0. Scope for blockhash: [12964744 - 12965000[
{"pc":28,"op":89,"gas":"0x88fc","gasCost":"0x2","memSize":32,"stack":["0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b","0x0","0x0","0x20","0x18","0x0"],"depth":97,"refund":0,"opName":"MSIZE"}

In this case, blocks [12964744 - 12965000[ are eligible, and the test queries BLOCKHASH(0). Which is not part of the scope.

The geth implementation is here: https://github.com/ethereum/go-ethereum/blob/master/core/vm/instructions.go#L435 - it checks the eligible span of blocks before actually trying to "resolve" the hashes (via keccak-cheat or disk lookup)