BLOCKHASH tests

[OlivierBBB]

We require tests for BLOCKHASH testing certain limiting + multiblock behaviour. Two blocks ought to be enough.

Multi block tests

Block becomes available

// Block 1: execute
NUMBER
BLOCKHASH

// Block 2: execute
PUSH1 1
NUMBER
SUB
BLOCKHASH

Block no longer available

// Block no longer available
// Block 1: execute
PUSH2 256
NUMBER
SUB
BLOCKHASH

// Block 2: execute
PUSH2 257
NUMBER
SUB
BLOCKHASH

Mono block tests

We maintain the tests written by @letypequividelespoubelles for the single block stuff. We add to it (I don't know how) tests around conflations containing the Genesis block.