fix invalid MinerPayouts in testutil.MineBlock

[peterjan]

Silly little bug in our MineBlock implementation where we call PoolTransactions twice and thus introduce a race. This eventually results in an error upon validating the block that indicates the miner payout sum doesn't match block reward + fees.

[lukechampine]

we should support V2 transactions while we're at it.

Also, I spot another race: the state when you call cm.PoolTransactions might not match the state when you called cm.TipState! That's annoying. We can squash it with a loop:

retry:
    cs := cm.TipState()
    txns := cm.PoolTransactions()
    txnsv2 := cm.PoolTransactionsV2()
    if cs.Index != cm.Tip() {
        goto retry
    }

But it might make more sense to add a helper function to chain.Manager that returns a state and the pool transactions together.

Alternatively, maybe this isn't a big deal? Miners already have to assume that a new block might arrive while they're mining. So they need to regularly check the tip and restart anyway.

[peterjan]

Hm if we do all that we might as well drop this outdated impl. all together in favour of the one in coreutils package?