simulators/ethereum/engine: Fix PoW Re-Org Tests

[marioevz]

Currently in some of the tests which require a PoW mining + Re-Orgs for simulator ethereum/engine, some clients are sending the newly mined block as NewBlockMsg back to the PoW Producer.
This results in the PoW Producer thinking it's out of sync after we re-org back to a lower height block, and therefore abort mining.
Therefore, the PoW Producer never mines the expected amount of terminal blocks and the test is invalid.

Miner needs to be rewritten to handle the mining without relying on the general blockchain status, and produce more deterministic tests.

[marioevz]

FIxed in #622