lightning-loop regtest: fix incorrectly succeeding test

When 'loop getparams' fails, jq gets no stdin and exits with code 0.
Because -o pipefail is not enabled in the testing shell, the whole test
command succeeds, although it should fail.

Just test "loop getparams" instead and ignore its output.

test/tests.py

@@ -337,7 +337,7 @@ def _():
         machine.wait_until_succeeds(
             log_has_string("lightning-loop", "Starting event loop at height 10")
         )
-        succeed("sudo -u operator loop getparams | jq -e '.rules'")
+        succeed("sudo -u operator loop getparams")
 
 
 if "netns-isolation" in enabled_tests: