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_bip68_sequence.py

@@ -327,7 +327,7 @@ class BIP68Test(BitcoinTestFramework):
         for i in range(2):
             block = create_block(tmpl=tmpl, ntime=cur_time)
             block.solve()
-            tip = block.sha256
+            tip = block.hash_int
             assert_equal(None if i == 1 else 'inconclusive', self.nodes[0].submitblock(block.serialize().hex()))
             tmpl = self.nodes[0].getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)
             tmpl['previousblockhash'] = '%x' % tip