[test] Fix wait condition in disconnect_p2ps

MY_SUBVERSION is defined in messages.py as a byte string, but here we were
comparing this value to the value returned by the RPC. Convert to ensure the
types match.

test/functional/test_framework/test_node.py

@@ -544,7 +544,7 @@ class TestNode():
 
     def num_test_p2p_connections(self):
         """Return number of test framework p2p connections to the node."""
-        return len([peer for peer in self.getpeerinfo() if peer['subver'] == MY_SUBVERSION])
+        return len([peer for peer in self.getpeerinfo() if peer['subver'] == MY_SUBVERSION.decode("utf-8")])
 
     def disconnect_p2ps(self):
         """Close all p2p connections to the node."""