message: [test] Manage node connections better in mempool persist test
  there were two calls to disconnect_nodes that were no-ops. fixed one & removed
  the other & added assertions to confirm node has no connections when creating
  the unbroadcast transaction.
diff: @@ -84,7 +84,9 @@ class MempoolPersistTest(BitcoinTestFramework):
  assert_greater_than_or_equal(tx_creation_time_higher, tx_creation_time)
  # disconnect nodes & make a txn that remains in the unbroadcast set.
  - disconnect_nodes(self.nodes[0], 2)
  + disconnect_nodes(self.nodes[0], 1)
  + assert(len(self.nodes[0].getpeerinfo()) == 0)
  + assert(len(self.nodes[0].p2ps) == 0)
  self.nodes[0].sendtoaddress(self.nodes[2].getnewaddress(), Decimal("12"))
  connect_nodes(self.nodes[0], 2)
  @@ -157,8 +159,10 @@ class MempoolPersistTest(BitcoinTestFramework):
  # clear out mempool
  node0.generate(1)
  - # disconnect nodes to make a txn that remains in the unbroadcast set.
  - disconnect_nodes(node0, 1)
  + # ensure node0 doesn't have any connections
  + # make a transaction that will remain in the unbroadcast set
  + assert(len(node0.getpeerinfo()) == 0)
  + assert(len(node0.p2ps) == 0)
  node0.sendtoaddress(self.nodes[1].getnewaddress(), Decimal("12"))
  # shutdown, then startup with wallet disabled
  