test: add external signer test

Includes a mock to mimick the HWI interace.

test/functional/wallet_signer.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python3
+# Copyright (c) 2017-2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test external signer.
+
+Verify that a bitcoind node can use an external signer command
+"""
+import os
+import platform
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+    assert_raises_rpc_error,
+)
+
+
+class SignerTest(BitcoinTestFramework):
+    def mock_signer_path(self):
+        path = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'mocks', 'signer.py')
+        if platform.system() == "Windows":
+            return "py " + path
+        else:
+            return path
+
+    def set_test_params(self):
+        self.num_nodes = 3
+
+        self.extra_args = [
+            [],
+            [f"-signer={self.mock_signer_path()}", '-keypool=10'],
+            [f"-signer={self.mock_signer_path()}", '-keypool=10'],
+            ["-signer=fake.py"],
+        ]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+        self.skip_if_no_external_signer()
+
+    def set_mock_result(self, node, res):
+        with open(os.path.join(node.cwd, "mock_result"), "w", encoding="utf8") as f:
+            f.write(res)
+
+    def clear_mock_result(self, node):
+        os.remove(os.path.join(node.cwd, "mock_result"))
+
+    def run_test(self):
+        self.log.debug(f"-signer={self.mock_signer_path()}")
+
+if __name__ == '__main__':
+    SignerTest().main()