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_maxuploadtarget.py

@@ -41,7 +41,7 @@ class TestP2PConn(P2PInterface):
         pass
 
     def on_block(self, message):
-        self.block_receive_map[message.block.sha256] += 1
+        self.block_receive_map[message.block.hash_int] += 1
 
 class MaxUploadTest(BitcoinTestFramework):