Clear TestNode objects after shutdown

TestNode objects need to be removed during shutdown, as setup_nodes does not
remove previous TestNode objects from previous test runs during setup.

Author: jachiang

test/functional/test_framework/test_framework.py

@@ -280,6 +280,8 @@ def shutdown(self):
             rpc_logger.removeHandler(h)
         if cleanup_tree_on_exit:
             shutil.rmtree(self.options.tmpdir)
+
+        self.nodes.clear()
         return exit_code
 
     # Methods to override in subclass test scripts.