resolve fixme in p2p_segwit.py functional test: the bad-witness-merkle-match reject reason is due to the Elements-style witness structure added to the serialisation of BrokenCTransaction in ElementsProject#494 causing the CheckWitnessMalleation check to fail before reaching CheckMerkleRoot which would fail with a bad-txnmrklroot reason

(cherry picked from commit 3039060)

Author: Tom Trevethan

test/functional/p2p_segwit.py

@@ -1223,7 +1223,7 @@ def serialize_with_witness(self):
 
         block = self.build_next_block()
         self.update_witness_block_with_transactions(block, [tx2])
-        test_witness_block(self.nodes[0], self.test_node, block, accepted=False, reason='bad-witness-merkle-match') # ELEMENTS: FIXME why is this reason different
+        test_witness_block(self.nodes[0], self.test_node, block, accepted=False, reason='bad-witness-merkle-match') # ELEMENTS: reason due to CheckWitnessMalleation and elements-style witness structure
 
         # Now try using a too short vtxinwit
         tx2.wit.vtxinwit.pop()