test: fix intermittent failure in wallet_reorgsrestore.py

Wait until the node's process has fully stopped before starting a new instance.

Since the same code is used in tool_wallet.py, this consolidates the behavior
into a 'kill_process()' function.

test/functional/test_framework/test_node.py

@@ -443,6 +443,11 @@ class TestNode():
             kwargs["expected_ret_code"] = 1 if expect_error else 0  # Whether node shutdown return EXIT_FAILURE or EXIT_SUCCESS
         self.wait_until(lambda: self.is_node_stopped(**kwargs), timeout=timeout)
 
+    def kill_process(self):
+        self.process.kill()
+        self.wait_until_stopped(expected_ret_code=1 if platform.system() == "Windows" else -9)
+        assert self.is_node_stopped()
+
     def replace_in_config(self, replacements):
         """
         Perform replacements in the configuration file.