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/p2p_getdata.py

@@ -19,7 +19,7 @@ class P2PStoreBlock(P2PInterface):
         self.blocks = defaultdict(int)
 
     def on_block(self, message):
-        self.blocks[message.block.sha256] += 1
+        self.blocks[message.block.hash_int] += 1
 
 
 class GetdataTest(BitcoinTestFramework):