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_headers_sync_with_minchainwork.py

@@ -129,7 +129,7 @@ class RejectLowDifficultyHeadersTest(BitcoinTestFramework):
             block = create_block(hashprev = hashPrevBlock, tmpl=node.getblocktemplate(NORMAL_GBT_REQUEST_PARAMS))
             block.solve()
             new_blocks.append(block)
-            hashPrevBlock = block.sha256
+            hashPrevBlock = block.hash_int
 
         headers_message = msg_headers(headers=new_blocks)
         p2p.send_and_ping(headers_message)