Add zarr-level coverage for v0.4 & v0.5

[DragaDoncila]

Currently we only have zarr-level tests for v0.6.

We should look at whether we can generate zarr tests from the existing attributes tests reliably, and/or hand-write any we think are critical.

[clbarnes]

This is how I created the zarr-level tests for 0.4 and 0.5 (which is why lots of them have dangling references a la #17 ). The dangle is the main issue with it!