pytest: fix spurious error in test_important_plugin

```
**BROKEN** plugin-spenderp: Lost connection to the RPC socket.
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_plugin.py

@@ -2239,7 +2239,7 @@ def test_important_plugin(node_factory):
     n = node_factory.get_node(options={"important-plugin": os.path.join(pluginsdir, "nonexistent")},
                               may_fail=True, expect_fail=True,
                               # Other plugins can complain as lightningd stops suddenly:
-                              broken_log='Plugin marked as important, shutting down lightningd|Reading JSON input: Connection reset by peer',
+                              broken_log='Plugin marked as important, shutting down lightningd|Reading JSON input: Connection reset by peer|Lost connection to the RPC socket',
                               start=False)
 
     n.daemon.start(wait_for_initialized=False, stderr_redir=True)