test: Replace use of (dis)?connect_nodes globals

A later scripted-diff commit replaces the majority of uses, which all
follow this pattern:

    (dis)?connect_nodes(self.nodes[a], b)

This commit replaces the few "special cases".

test/functional/mempool_unbroadcast.py

@@ -11,9 +11,7 @@ from test_framework.p2p import P2PTxInvStore
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
-    connect_nodes,
     create_confirmed_utxos,
-    disconnect_nodes,
 )
 
 MAX_INITIAL_BROADCAST_DELAY = 15 * 60 # 15 minutes in seconds
@@ -36,7 +34,7 @@ class MempoolUnbroadcastTest(BitcoinTestFramework):
         min_relay_fee = node.getnetworkinfo()["relayfee"]
         utxos = create_confirmed_utxos(min_relay_fee, node, 10)
 
-        disconnect_nodes(node, 1)
+        self.disconnect_nodes(0, 1)
 
         self.log.info("Generate transactions that only node 0 knows about")
 
@@ -70,7 +68,7 @@ class MempoolUnbroadcastTest(BitcoinTestFramework):
         self.restart_node(0)
 
         self.log.info("Reconnect nodes & check if they are sent to node 1")
-        connect_nodes(node, 1)
+        self.connect_nodes(0, 1)
 
         # fast forward into the future & ensure that the second node has the txns
         node.mockscheduler(MAX_INITIAL_BROADCAST_DELAY)
@@ -91,7 +89,7 @@ class MempoolUnbroadcastTest(BitcoinTestFramework):
         time.sleep(2) # allow sufficient time for possibility of broadcast
         assert_equal(len(conn.get_invs()), 0)
 
-        disconnect_nodes(node, 1)
+        self.disconnect_nodes(0, 1)
         node.disconnect_p2ps()
 
     def test_txn_removal(self):

test/functional/rpc_psbt.py

@@ -13,7 +13,6 @@ from test_framework.util import (
     assert_greater_than,
     assert_raises_rpc_error,
     connect_nodes,
-    disconnect_nodes,
     find_output,
 )
 
@@ -46,7 +45,7 @@ class PSBTTest(BitcoinTestFramework):
 
         # Disconnect offline node from others
         # Topology of test network is linear, so this one call is enough
-        disconnect_nodes(offline_node, 1)
+        self.disconnect_nodes(0, 1)
 
         # Create watchonly on online_node
         online_node.createwallet(wallet_name='wonline', disable_private_keys=True)