layout of tests

What is your issue?

Currently the tests are stored in a flat on-disk layout, but some test files are logically grouped, e.g. the relationship between test_backends_api.py and test_backends_common.py could be expressed via a directory structure, like test/backends/test_api.py and test/backends/test_common.py.

Relatedly, some of the test files are a bit long -- test_backends.py is 6500 lines, and it contains tests for zarr and netcdf (and maybe other things). If someone is just working on the zarr side of things, then the netcdf tests are thousands of lines of clutter that could in principle be entirely contained in a separate test file.

So my proposal would be to judiciously group logically related tests into modules , e.g. test_backends would be one, and also split large test files into smaller independent components, e.g. test_backends/test_zarr.py would just test the zarr backend stuff.

Does this seem reasonable?

Start by reviewing test_backends.py and the related test_backends_api.py and test_backends_common.py files. Map the logical groupings and backend-specific sections before proposing directory layouts and smaller test modules. Done means the tests are organized into focused components while preserving their existing coverage.