test(state): report mgas/s metric in EVM benchmark

[shekhirin]

I think it's good to have mgas/s metric in EVM benchmarks, so anyone could compare geth's implementation with e.g. evmone's which has the same reporting format.

Ideally, we would be able to have all EVM implementations benchmarked against one metric that's comparable regardless of implementation language.

[shekhirin]

@MariusVanDerWijden agree, fixed!

[holiman]

After looking into it a bit more, I have fixed the refunds in the benchmarks now. As for

This error is also present in core/blockchain_insert.go, ... but we should fix it in both places, rather than do it erroneously here too.

I no longer believe that to be the case. But the flow is a bit hard to follow, so I need to verify that experimentally. If it needs to be fixed, it should be fixed in a separate PR, since it touches some core stuff.