test: Fix race condition in p2p_txexpiringsoon

The recent changes to mempool inv logic mean that nodes are much less
likely to immediately return an `inv` message in response to a `mempool`
message. The `p2p_txexpiringsoon` RPC test was relying on the prior
behaviour.

`TestNode.sync_with_ping` now takes an optional `waiting_for` closure
that allows the caller to require that a specific message kind is
received prior to the timeout.

qa/rpc-tests/p2p_txexpiringsoon.py

@@ -51,7 +51,7 @@ class TxExpiringSoonTest(BitcoinTestFramework):
             testnode.send_message(msg_mempool())
 
         # Sync up with node after p2p messages delivered
-        testnode.sync_with_ping()
+        testnode.sync_with_ping(waiting_for=lambda x: x.last_inv)
 
         with mininode_lock:
             msg = testnode.last_inv

qa/rpc-tests/tx_expiry_helper.py

@@ -66,12 +66,13 @@ class TestNode(NodeConnCB):
 
     # The following function is mostly copied from p2p-acceptblock.py
     # Sync up with the node after delivery of a message
-    def sync_with_ping(self, timeout=30):
+    def sync_with_ping(self, timeout=30, waiting_for=None):
         self.connection.send_message(msg_ping(nonce=self.ping_counter))
         sleep_time = 0.05
         while timeout > 0:
             with mininode_lock:
-                if self.last_pong.nonce == self.ping_counter:
+                ready = True if waiting_for is None else waiting_for(self) is not None
+                if ready and self.last_pong.nonce == self.ping_counter:
                     self.ping_counter += 1
                     return
             time.sleep(sleep_time)