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_versionbits_warning.py

@@ -50,7 +50,7 @@ class VersionBitsWarningTest(BitcoinTestFramework):
             peer.send_without_ping(msg_block(block))
             block_time += 1
             height += 1
-            tip = block.sha256
+            tip = block.hash_int
         peer.sync_with_ping()
 
     def versionbits_in_alert_file(self):