This removes the method `TestingTTDBlock` introduced by #30744. It was
added to make the beacon consensus engine aware of the merge block in
tests without relying on the total difficulty. However, tracking the
merge block this way is very annoying. We usually configure forks in the
`ChainConfig`, but the method is on the consensus engine, which isn't
always created in the same place. By sidestepping the `ChainConfig` we
don't get the usual fork-order checking, so it's possible to enable the
merge before the London fork, for example. This in turn can lead to very
hard-to-debug outputs and validation errors.
So here I'm changing the consensus engine to check the
`MergeNetsplitBlock` instead. Alternatively, we assume a network is
merged if it has a `TerminalTotalDifficulty` of zero, which is a very
common configuration in tests.
9064038a86