[PATCH] tests: add basic smoke/integration test script

[Aayushmaan Chakraborty]

Hi Gao Xiang and linux-erofs team,

The repository currently lacks any automated smoke or integration tests.

This patch adds a simple, self-contained bash script (tests/smoke.sh)
that verifies the primary userspace workflow:

- mkfs.erofs image creation (uncompressed)
- fsck.erofs integrity check
- erofsfuse FUSE mount
- Basic content verification
- Clean unmount and full cleanup

The script:
- Checks for required binaries upfront with a helpful error message
- Fails fast with clear diagnostics
- Uses trap for cleanup on exit or failure
- Exits non-zero on failure

Happy to expand it (e.g., add compressed image variant, error
injection, or CI integration).

Thanks for considering!

Best regards,
Aayushmaan Chakraborty
GitHub: https://github.com/Aayushmaan-24/erofs-utils/tree/add-basic-smoke-test

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