test: simplify (w)txid checks by avoiding .calc_sha256 calls

Calls on the tx.calc_sha256 method can be confusing, as they return
the result (either txid or wtxid, depending on the with_witness
boolean parameter) as integer rather than as actual (w)txid. Use
.rehash() and .getwtxid() instead to improve readability and in some
cases avoid a conversion from string-txid to an integer.

test/functional/p2p_segwit.py

@@ -336,8 +336,7 @@ class SegWitTest(BitcoinTestFramework):
 
         # Verify the hash with witness differs from the txid
         # (otherwise our testing framework must be broken!)
-        tx.rehash()
-        assert tx.sha256 != tx.calc_sha256(with_witness=True)
+        assert tx.rehash() != tx.getwtxid()
 
         # Construct a block that includes the transaction.
         block = self.build_next_block()
@@ -1293,7 +1292,7 @@ class SegWitTest(BitcoinTestFramework):
         # Test that getrawtransaction returns correct witness information
         # hash, size, vsize
         raw_tx = self.nodes[0].getrawtransaction(tx3.hash, 1)
-        assert_equal(int(raw_tx["hash"], 16), tx3.calc_sha256(True))
+        assert_equal(raw_tx["hash"], tx3.getwtxid())
         assert_equal(raw_tx["size"], len(tx3.serialize_with_witness()))
         vsize = tx3.get_vsize()
         assert_equal(raw_tx["vsize"], vsize)