test: fix another intermittent failure in wallet_basic.py

During init, the test framework will start using rpc after the
mempool was loaded. It will not wait for postInitProcess or
outstanding transactionAddedToMempool notifications, leading to
a possible race, in which listunspent is being called while the
tx is still in Inactive status. Prevent this by processing
outstanding notifications.

Author: mzumsande

test/functional/wallet_basic.py

@@ -656,6 +656,9 @@ def run_test(self):
 
         # check that it works again with -spendzeroconfchange set (=default)
         self.restart_node(0, ["-spendzeroconfchange=1"])
+        # Make sure the wallet knows the tx in the mempool
+        self.nodes[0].syncwithvalidationinterfacequeue()
+
         zeroconf_wallet = self.nodes[0].get_wallet_rpc("zeroconf")
         utxos = zeroconf_wallet.listunspent(minconf=0)
         assert_equal(len(utxos), 1)