Merge bitcoin/bitcoin#23365: index: Fix backwards search for bestblock

9600ea01450b0d39be90eb2971c1ac5c9b69a66e test: Add edge case of pruning up to index height (Martin Zumsande)
698c524698c33595a4d555eaa9e21bc19b4d3e93 index: Fix backwards search for bestblock (Martin Zumsande)

Pull request description:

  This PR attempts to fix an intermittent Init issue encountered during the stress testing of #23289, which relates to the pruning-compatible filter reconstruction logic introduced in #15946.

  The problem would occur when the node starts with `-txindex=1` but `ThreadSync` is interrupted after it sets `m_best_block_index` to Genesis, and before it gets do any further work.
  In that case, during the next restart of the node, an Init error would be thrown because  `BaseIndex::Init()` tries to backtrack from the tip to the last block which has been successfully indexed (here: Genesis), but the backtracking logic didn't work properly in this case:
  The loop
  `while (block_to_test && block->pprev && (block->pprev->nStatus & BLOCK_HAVE_DATA))` checks if a predecessor exists **before** performing the check `block_to_test == block` and then possbily setting `prune_violation = false`
  If `block_to_test` and `block` are the Genesis block this check will not be reached because `block->pprev` does not exist.

  To reproduce this bug on regtest:
  1) start a node with a fresh datadir using `-txindex=1` (or any other index)
  2) stop and restart without any index
  3) mine a block
  3) stop and restart again with the index enabled
  ->InitError `Error: txindex best block of the index goes beyond pruned data. (...)`

  Fix this by requiring that we have the data for the block of the current iteration `block` (instead of requiring it for the predecessor `block->pprev`)
  That way, the check for `block_to_test == block` is also reached when `block_to_test` is the Genesis block.
  No longer requiring the data of `block->pprev` also means that we can now prune up to `m_best_block_index` height without requiring a reindex (one block more than before). I added this edge case to `feature_blockfilterindex_prune.py`, the new version should fail on master.

ACKs for top commit:
  ryanofsky:
    Partial code review ACK 9600ea01450b0d39be90eb2971c1ac5c9b69a66e for the code change, not the test changes. (Test changes are indirect and little over my head.) It seems obvious that previous code `prune_violation = true, while (block->pprev)` would incorrectly detect a prune violation at the genesis block, and the fix here make sense and looks correct.

Tree-SHA512: c717f372cee8fd49718b1b18bfe237aa6ba3ff4468588c10e1272d7a2ef3981d10af4e57de51dec295e2ca72d441bc6c2812f7990011a94d7f818775e3ff1a38

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