[backport#15991] Bugfix: fix pruneblockchain returned prune height
Summary:
fixup: Fix prunning test (João Barbosa)
Fix RPC/pruneblockchain returned prune height (Jonas Schnelli)
Pull request description:
The help of `pruneblockchain` tells us that the return value is `Height of the last block pruned.`,... but the implementation naively returns the provided input `height` and therefore not respecting that pruning can't be done on all possible blockheight due to the fact that we only prune complete blockfiles (which combine multiple blocks). This fixes the return value to actually return the correct prune height.
Backport of Core PR15991
Test Plan:
ninja check check-functional
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7389