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/mempool_updatefromblock.py

@@ -44,7 +44,7 @@ class MempoolUpdateFromBlockTest(BitcoinTestFramework):
             block = create_block(tip, create_coinbase(height + 1), block_time)
             block.solve()
             blocks.append(block)
-            tip = block.sha256
+            tip = block.hash_int
             block_time += 1
             height += 1