[test] Check user agent string from test framework connections

Add a check that new connections from the test framework to the
node have the correct user agent string. This makes bugs easier
to detect if the user agent string ever changes.

test/functional/test_framework/test_node.py

@@ -26,6 +26,7 @@
 from .p2p import P2P_SUBVERSION
 from .util import (
     MAX_NODES,
+    assert_equal,
     append_config,
     delete_cookie_file,
     get_auth_cookie,
@@ -545,6 +546,11 @@ def add_p2p_connection(self, p2p_conn, *, wait_for_verack=True, **kwargs):
             # in comparison to the upside of making tests less fragile and unexpected intermittent errors less likely.
             p2p_conn.sync_with_ping()
 
+            # Consistency check that the Bitcoin Core has received our user agent string. This checks the
+            # node's newest peer. It could be racy if another Bitcoin Core node has connected since we opened
+            # our connection, but we don't expect that to happen.
+            assert_equal(self.getpeerinfo()[-1]['subver'], P2P_SUBVERSION)
+
         return p2p_conn
 
     def add_outbound_p2p_connection(self, p2p_conn, *, p2p_idx, connection_type="outbound-full-relay", **kwargs):