test: add external signer test

Includes a mock to mimick the HWI interace.

test/functional/mocks/signer.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+# Copyright (c) 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.
+
+import os
+import sys
+import argparse
+import json
+
+def perform_pre_checks():
+    mock_result_path = os.path.join(os.getcwd(), "mock_result")
+    if(os.path.isfile(mock_result_path)):
+        with open(mock_result_path, "r", encoding="utf8") as f:
+            mock_result = f.read()
+        if mock_result[0]:
+            sys.stdout.write(mock_result[2:])
+            sys.exit(int(mock_result[0]))
+
+parser = argparse.ArgumentParser(prog='./signer.py', description='External signer mock')
+subparsers = parser.add_subparsers(description='Commands', dest='command')
+subparsers.required = True
+
+args = parser.parse_args()
+
+perform_pre_checks()
+
+args.func(args)