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_csv_activation.py

@@ -165,7 +165,7 @@ class BIP68_112_113Test(BitcoinTestFramework):
             block = self.create_test_block([])
             test_blocks.append(block)
             self.last_block_time += 600
-            self.tip = block.sha256
+            self.tip = block.hash_int
             self.tipheight += 1
         return test_blocks