fix: assert parent-side NsJail startup failure in supervisor test #317

The supervisor regression test added by #315 fails on both Linux amd64 and arm64. tests/supervisor_policy.py:50 expects Launching child process failed, but the parent reports Received error message from the child process before it has been executed and a child-launch failure. The child is then terminated with SIGKILL, so its final log line is not guaranteed to appear.

The preceding syscall-denial and TCP checks pass. The missing executable produces a nonzero exit and the expected parent-side error notification, so this is a log assertion regression, not evidence that the startup handshake succeeded incorrectly.

Please assert the parent-side handshake failure diagnostic instead of requiring the child-side final log. Keep the nonzero-exit assertion. This blocks the native supervisor CI job before the remaining stdio/DNS/hosted-network tests run.

Reproduction: build the nsjail-ipc-test target and run python3 /tests/supervisor_policy.py with the test directory mounted and Docker seccomp unconfined, as in the CI workflow.