pytest: fix plugin fail timeout test:

We didn't update this when we extended the timeout to 120 seconds in
ee3133f ("lightningd: increase
startup time for plugins to 120 seconds.")

```
    def test_failing_plugins(directory):
        fail_plugins = [
            os.path.join(os.getcwd(), 'contrib/plugins/fail/failtimeout.py'),
            os.path.join(os.getcwd(), 'contrib/plugins/fail/doesnotexist.py'),
        ]
    
        for p in fail_plugins:
>           with pytest.raises(subprocess.CalledProcessError):
E           Failed: DID NOT RAISE <class 'subprocess.CalledProcessError'>

tests/test_plugin.py:420: Failed
----------------------------- Captured stdout call -----------------------------
{'github_repository': 'ElementsProject/lightning', 'github_sha': '83dca18c5e9610bfaac766f957387b9a1ec48f50', 'github_ref': 'refs/pull/7887/merge', 'github_ref_name': 'HEAD', 'github_run_id': 13253210143, 'github_head_ref': 'guilt/bolt-updates-after-24.11', 'github_run_number': 12237, 'github_base_ref': 'master', 'github_run_attempt': '2', 'testname': 'test_failing_plugins', 'start_time': 1739239278, 'end_time': 1739239340, 'outcome': 'fail'}
=========================== short test summary info ============================
FAILED tests/test_plugin.py::test_failing_plugins - Failed: DID NOT RAISE <class 'subprocess.CalledProcessError'>
============= 1 failed, 80 passed, 2 skipped in 855.37s (0:14:15) ==============
```

Author: rustyrussell

contrib/plugins/fail/failtimeout.py

@@ -9,8 +9,8 @@
 
 
 def json_getmanifest(request, **kwargs):
-    # Timeout is 60 seconds, so wait more
-    time.sleep(61)
+    # Timeout is 120 seconds, so wait more
+    time.sleep(121)
     return {
         "options": [
         ],