test: get and decode tx with a single gettransaction RPC call

Rather than subsequently calling `gettransaction` and
`decoderawtransaction` to get the decoded information  for a specific
tx-id, we can simply use the verbose version of `gettransaction`, which
returns this in a 'decoded' key. I.e.

node.decoderawtransaction(node.gettransaction(txid)['hex'])

can be replaced by:

node.gettransaction(txid=txid, verbose=True)['decoded']

test/functional/mempool_packages.py

@@ -65,8 +65,7 @@ class MempoolPackagesTest(BitcoinTestFramework):
             value = sent_value
             chain.append(txid)
             # We need the wtxids to check P2P announcements
-            fulltx = self.nodes[0].getrawtransaction(txid)
-            witnesstx = self.nodes[0].decoderawtransaction(fulltx, True)
+            witnesstx = self.nodes[0].gettransaction(txid=txid, verbose=True)['decoded']
             witness_chain.append(witnesstx['hash'])
 
             # Check that listunspent ancestor{count, size, fees} yield the correct results