Fix sandbox tests for Linux: handle different shell path and error message

- Handle /usr/bin/bash (Linux) vs /bin/bash (macOS) in /tmp write test
- Handle 'Read-only file system' (Linux) vs 'Operation not permitted' (macOS)
- Add 'Read-only file system' to outputLooksSandboxBlocked heuristic
- Replace newlines with spaces (not empty) to handle terminal wrapping
- Extract outputLooksSandboxBlocked as exported function with unit tests