fn test_writer(n_bufs: usize, per_call: usize) -> TestWriter {
        TestWriter { n_bufs, per_call, written: Vec::new() }
    }
Create a new writer that reads from at most `n_bufs` and reads
`per_call` bytes (in total) per call to write.
futures-util/src/io/write_all_vectored.rs