Rust-only fixtures for getting array data

[ilan-gold]

We currently do not unit-test in rust because the calling python code can do this for us, but this can make development a bit slow because you have to recompile every time via maturin develop before running tests, which is error-pronce.

Another advantage of setting up "fixture" that look like the python requests is to be able to benchmark within rust only.

In other words we should create inputs to

zarrs-python/src/array.rs

Line 414 in d29343b

pub fn retrieve_chunk_subset(

as if they were actually coming from python, but purely in rust.