cmd/evm: slight change in how t8n handles coinbase pre eip-158

[holiman]

This PR fixes a subtle bug in t8n. After this PR, t8n behaves like our state-test runner in this particular scenario:

• After an empty block,
• And the config is that there is a zero block reward,
• And the coinbase did not previously exist,
• And we're before EIP-158,
• Then the coinbase should be touched, a.k.a brought into existence.

Our state test runner handles that here: https://github.com/ethereum/go-ethereum/blob/master/tests/state_test_util.go#L266

The t8n was meant to handle that exactly identical, but failed to do so. That's what's being fixed by this PR.
Also related, the miningReward comes from this flag:

	RewardFlag = &cli.Int64Flag{
		Name:  "state.reward",
		Usage: "Mining reward. Set to -1 to disable",
		Value: 0,
	}

cc @winsvega

[winsvega]

Would like to point out that besu client discovered that by test RPC