fix(strongswan-connections): helpful error when --test points at legacy fixture path

Two follow-up cleanups on top of the three-file-to-single-file
refactor in the previous commit:

1. Validate that the --test fixture file actually exists before
   feeding it to lib.lftest.test(), which otherwise silently
   returns the bogus path as stdout and leaves json.loads() to
   crash with an opaque "Expecting value" message.

2. Detect the three legacy suffixes (-possible_connection_keys,
   -active_connection_keys, -list_sas) on a missing path and
   include a pointer at the new single-file path in the error
   message, so anyone with a stale command from before the
   refactor gets a one-line "drop the suffix" hint instead of a
   generic FileNotFoundError.

Also wrap the json.loads() in try/except and surface the filename
on a JSONDecodeError so a corrupt fixture is no longer anonymous.

Author: markuslf

check-plugins/strongswan-connections/strongswan-connections

@@ -12,6 +12,7 @@
 
 import argparse
 import json
+import os
 import re
 import socket
 import sys
@@ -312,8 +313,27 @@ def main():
         # derived from `list_sas` below via the same `_collect_keys`
         # helper the production path uses, so the test and the
         # production paths cannot drift.
+        fixture_path = args.TEST[0] if args.TEST else ''
+        if not fixture_path or not os.path.isfile(fixture_path):
+            hint = ''
+            for legacy in (
+                '-possible_connection_keys',
+                '-active_connection_keys',
+                '-list_sas',
+            ):
+                if fixture_path.endswith(legacy):
+                    hint = (
+                        f' (the three-file convention was removed; drop the '
+                        f'`{legacy}` suffix and pass '
+                        f'`{fixture_path[: -len(legacy)]}` instead)'
+                    )
+                    break
+            lib.base.cu(f'Test fixture not found: {fixture_path}{hint}')
         stdout, _stderr, _retc = lib.lftest.test(args.TEST)
-        fixture = json.loads(stdout)
+        try:
+            fixture = json.loads(stdout)
+        except json.JSONDecodeError as e:
+            lib.base.cu(f'Malformed JSON in test fixture {fixture_path}: {e}')
         list_conns = fixture.get('list_conns', [])
         list_sas = fixture.get('list_sas', [])
     possible_connection_keys = _collect_keys(list_conns)