test: rename CBlockHeader .rehash()/.sha256 -> .hash_int for consistency

Note that we unfortunately can't use a scripted diff here, as the
`sha256` symbol is also used for other instances (e.g. as function
in hashlib, or in the `UTXO` class in p2p_segwit.py).

test/functional/feature_pruning.py

@@ -58,7 +58,7 @@ def mine_large_blocks(node, n):
         # Submit to the node
         node.submitblock(block.serialize().hex())
 
-        previousblockhash = block.sha256
+        previousblockhash = block.hash_int
         height += 1
         mine_large_blocks.nTime += 1