[tests] Allow stderr to be tested against specified string

Allow bitcoind's stderr to be tested against a specified string on
shutdown.

test/functional/test_framework/test_framework.py

@@ -289,9 +289,9 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
             for node in self.nodes:
                 coverage.write_all_rpc_commands(self.options.coveragedir, node.rpc)
 
-    def stop_node(self, i):
+    def stop_node(self, i, expected_stderr=''):
         """Stop a bitcoind test node"""
-        self.nodes[i].stop_node()
+        self.nodes[i].stop_node(expected_stderr)
         self.nodes[i].wait_until_stopped()
 
     def stop_nodes(self):