Add more unit tests for GPU buffer

[maxrjones]

This PR adds some tests for the GPU buffer prototype to unlock #2738.

[maxrjones]

@TomAugspurger would you expect the sharding codec to work with the gpu buffer prototype? If so, I could debug the failure more but don't want to spend time setting up an environment with cuda unnecessarily

[TomAugspurger]

I haven't used or looked at the sharding code, so I don't know offhand sorry.

If setting up a dev end with cupy is a hassle I can take a look sometime.

Just looking at the traceback, maybe one thing to check: try setting with zarr.config.enable_gpu() in the tests? Maybe we're grabbing a buffer type from the default config somewhere where we should be basing it off another Buffer class?

[jakirkham]

would you expect the sharding codec to work with the gpu buffer prototype?

cc @madsbk @akshaysubr (in case either of you have thoughts on this question)

[akshaysubr]

Its been a while since I looked at the sharding codec, but it was not expected to work with the GPU buffer prototype. Might be worth another closer look but it seemed like some things in the sharding codec need to still be generalized to work with arbitrary buffer prototypes.

[d-v-b]

the sharding codec is unique because it does IO, and thus interacts with a store, which might be the root of the complication here. (I would argue it is not really a codec in the conventional sense, and is rather a special implementation of an array).