test: add missing sync to p2p_tx_download.py

This prevents intermittent failures - if the node hasn't processed
the inv from the outbound peer before the mocktime bump, the peer
won't be preferred after the other inv timeouts, failing the test .
Therefore, add a sync, just like there is one after the send_message
calls in the previous lines.

Author: mzumsande

test/functional/p2p_tx_download.py

@@ -274,6 +274,7 @@ def test_preferred_tiebreaker_inv(self):
         pref_peer = self.nodes[0].add_outbound_p2p_connection(
            TestP2PConn(), wait_for_verack=True, p2p_idx=1, connection_type="outbound-full-relay")
         pref_peer.send_message(msg_inv([CInv(t=MSG_WTX, h=0xff00ff00)]))
+        pref_peer.sync_with_ping()
 
         assert_equal(len(self.nodes[0].getpeerinfo()), NUM_INBOUND + 2)