Simulator ethereum/engine: Difficulty bomb PoW Merge Tests

[marioevz]

There are two potential relevant test cases for the merge transition that have been identified, related to the difficulty bomb hitting mainnet before the merge happens:

The block that hits the TTD has a much higher difficulty than its parent

This test case would require that the clients somehow accept a block with a significant increase in difficulty.
An initial idea would be to create a fake hard-fork which pushes the difficulty bomb forward, so the bomb goes off immediately on the first blocks from genesis.

The block that hits the TTD is created much later than its parent

The TTD block takes a couple of minutes to be mined, this test would check that the CL keeps producing slots/blocks without issues in the meantime and that, when the TTD block is finally available, the merge happens, and blocks go back to 12-sec.
An initial idea would be to have a simulator testnet with PoW enabled, and having a separate miner which is capable of recognizing and withholding the TTD-reaching block for a couple of minutes before broadcasting it to clients.

[fjl]

We probably won't add those tests since the merge is now over.