(For newcomers:) more tests and benchmarks

We need more tests and benchmarks. They will help us debug and improve the code quality.

This issue is for newcomers of this project. If you would like to contribute, but don't know which work you can do, you can start with a test or a benchmark. The experience with `compio` will help you get familiar with the code.

See [monoio](https://github.com/bytedance/monoio) and [tokio-uring](https://github.com/tokio-rs/tokio-uring) for reference of tests.