[refactor] clarify tests by referencing p2p objects directly

Use object returned from add_p2p_connection to refer to
p2ps. Add a test class attribute if it needs to be used across
many methods. Don't use the p2p property.

test/functional/mempool_packages.py

@@ -58,7 +58,7 @@ class MempoolPackagesTest(BitcoinTestFramework):
 
     def run_test(self):
         # Mine some blocks and have them mature.
-        self.nodes[0].add_p2p_connection(P2PTxInvStore()) # keep track of invs
+        peer_inv_store = self.nodes[0].add_p2p_connection(P2PTxInvStore()) # keep track of invs
         self.nodes[0].generate(101)
         utxo = self.nodes[0].listunspent(10)
         txid = utxo[0]['txid']
@@ -80,7 +80,7 @@ class MempoolPackagesTest(BitcoinTestFramework):
 
         # Wait until mempool transactions have passed initial broadcast (sent inv and received getdata)
         # Otherwise, getrawmempool may be inconsistent with getmempoolentry if unbroadcast changes in between
-        self.nodes[0].p2p.wait_for_broadcast(witness_chain)
+        peer_inv_store.wait_for_broadcast(witness_chain)
 
         # Check mempool has MAX_ANCESTORS transactions in it, and descendant and ancestor
         # count and fees should look correct