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 https://github.com/ilan-gold/zarrs-python/blob/d29343b6ab2762e6dc7f0186217ecc8fe1f85fba/src/array.rs#L414 as if they were actually coming from python, but purely in rust.