test: check for specific disconnect reasons in feature_maxuploadtarget.py

This ensures that the disconnect happens for the expected reason and
also makes it easier to navigate between implementation and test code,
i.e. both the questions "do we have test coverage for this disconnect?"
(from an implementation reader's perspective) and "where is the code
handling this disconnect?" (from a test reader's perspective) can be
answered simply by grep-ping the corresponding debug message.

Author: theStack

test/functional/feature_maxuploadtarget.py

@@ -119,9 +119,10 @@ def run_test(self):
         assert_equal(len(self.nodes[0].getpeerinfo()), 3)
         # At most a couple more tries should succeed (depending on how long
         # the test has been running so far).
-        for _ in range(3):
-            p2p_conns[0].send_message(getdata_request)
-        p2p_conns[0].wait_for_disconnect()
+        with self.nodes[0].assert_debug_log(expected_msgs=["historical block serving limit reached, disconnect peer"]):
+            for _ in range(3):
+                p2p_conns[0].send_message(getdata_request)
+            p2p_conns[0].wait_for_disconnect()
         assert_equal(len(self.nodes[0].getpeerinfo()), 2)
         self.log.info("Peer 0 disconnected after downloading old block too many times")
 
@@ -143,8 +144,9 @@ def run_test(self):
 
         # But if p2p_conns[1] tries for an old block, it gets disconnected too.
         getdata_request.inv = [CInv(MSG_BLOCK, big_old_block)]
-        p2p_conns[1].send_message(getdata_request)
-        p2p_conns[1].wait_for_disconnect()
+        with self.nodes[0].assert_debug_log(expected_msgs=["historical block serving limit reached, disconnect peer"]):
+            p2p_conns[1].send_message(getdata_request)
+            p2p_conns[1].wait_for_disconnect()
         assert_equal(len(self.nodes[0].getpeerinfo()), 1)
 
         self.log.info("Peer 1 disconnected after trying to download old block")