qa: correct reorg_shit function to use given height

It was using the passed height for all other calculation, except for the
invalidation itself where it was using the wider-scope-initial-tip! This
is probably a mistake due to moving it to its own, inner, function.

tests/test_chain.py

@@ -208,7 +208,7 @@ def reorg_shift(height, txs):
  """Remine the chain from given height, shifting the txs by one block."""
  delta = bitcoind.rpc.getblockcount() - height + 1
  assert delta > 2
- h = bitcoind.rpc.getblockhash(initial_tip["height"])
+ h = bitcoind.rpc.getblockhash(height)
  bitcoind.rpc.invalidateblock(h)
  bitcoind.generate_block(1)
  for tx in txs: