Test reorgs of the binary trie post flip

## Observation

At some block N, we would switch from the MPT to the PBT. Ideally nodes do not drop all MPT related state because if we reorg back to block N-1 we will be at the MPT block again. Nodes should in the worse case drop state after block N has been finalised.

## Suggestion

This is a tracking issue for testing this reorg edge case

Start by locating the MPT/PBT transition logic and existing reorganization tests in the execution specs. Exercise a reorg from the PBT transition block back to the preceding MPT block, then verify state is retained until the transition block is finalized; done means this edge case is covered by a passing test.

Written by the indexing model from the issue text.