Improve registry testing of included codecs

[jakirkham]

See discussion starting with this comment ( #347 (comment) ).

cc @joshmoore @rabernat @MSanKeys963 (for awareness)

[joshmoore]

See #349 for the detection part of this. The next step would be adding additional checks for each of the codecs. Probably the most important is that a fixture exists for each codec:

https://github.com/zarr-developers/numcodecs/tree/main/fixture