test: detect no external signer connected

test/functional/mocks/no_signer.py

@@ -0,0 +1,29 @@
+#!/usr/bin/env python3
+# Copyright (c) 2025-present 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 argparse
+import json
+import sys
+
+def enumerate(args):
+    sys.stdout.write(json.dumps([]))
+
+parser = argparse.ArgumentParser(prog='./no_signer.py', description='No external signer connected mock')
+
+subparsers = parser.add_subparsers(description='Commands', dest='command')
+subparsers.required = True
+
+parser_enumerate = subparsers.add_parser('enumerate', help='list available signers')
+parser_enumerate.set_defaults(func=enumerate)
+
+
+if not sys.stdin.isatty():
+    buffer = sys.stdin.read()
+    if buffer and buffer.rstrip() != "":
+        sys.argv.extend(buffer.rstrip().split(" "))
+
+args = parser.parse_args()
+
+args.func(args)