nixos/tests/corerad: Use curl --fail

Changed files
+1 -1
nixos
tests
+1 -1
nixos/tests/corerad.nix
···
), "SLAAC temporary address was not configured on client after router advertisement"
with subtest("Verify HTTP debug server is configured"):
-
out = router.succeed("curl localhost:9430/metrics")
+
out = router.succeed("curl -f localhost:9430/metrics")
assert (
"corerad_build_info" in out