Keep trying to receive websocket notification in test

This may or may not fix #1057 and other test failures. Since the network
delay in receiving the message is unknown (but expected to be very
small, usually), we keep trying to receive notification of the
transaction message in the websocket test 10 times for a total of 2
seconds (on the assumption that if it takes longer than that, something
else is wrong).

jmclient/test/test_wallet_rpc.py

@@ -169,7 +169,8 @@ def test_notif(self):
         # create a small delay between the instruction to send
         # the notification, and the checking of its receipt,
         # otherwise the client will be queried before the notification
-        # arrived:
+        # arrived. We will try a few times before giving up.
+        self.attempt_receipt_counter = 0
         d.addCallback(self.wait_to_receive)
         return d
 
@@ -178,7 +179,13 @@ def wait_to_receive(self, res):
         return d
 
     def checkNotifs(self):
-        assert self.client_factory.notifs == 1
+        if self.attempt_receipt_counter > 10:
+            assert False
+        if not self.client_factory.notifs == 1:
+            jlog.info("Failed to receive notification, waiting and trying again")
+            self.attempt_receipt_counter += 1
+            d = task.deferLater(reactor, 0.2, self.checkNotifs)
+            return d
 
     def fire_tx_notif(self):
         self.daemon.wss_factory.sendTxNotification(self.test_tx,