test: refactor: take use of create_block txlist parameter

Passing a list of transactions `txlist` to `create_block` appends
them to the block, hence we don't need to do that manually anymore.
The merkle root calculation can also be removed, since this is done
in the end of the helper.

test/functional/feature_csv_activation.py

@@ -173,9 +173,7 @@ class BIP68_112_113Test(BitcoinTestFramework):
         return test_blocks
 
     def create_test_block(self, txs):
-        block = create_block(self.tip, create_coinbase(self.tipheight + 1), self.last_block_time + 600)
-        block.vtx.extend(txs)
-        block.hashMerkleRoot = block.calc_merkle_root()
+        block = create_block(self.tip, create_coinbase(self.tipheight + 1), self.last_block_time + 600, txlist=txs)
         block.solve()
         return block