Rename Python directory test to tests

[jakirkham]

...we should consider making cuvs match the rest of RAPIDS and use the name "tests" for this folder.

See:

• https://github.com/rapidsai/cudf/tree/branch-25.02/python/cudf/cudf/tests
• https://github.com/rapidsai/rmm/tree/branch-25.02/python/rmm/rmm/tests
• https://github.com/rapidsai/cuml/tree/branch-25.02/python/cuml/cuml/tests
• https://github.com/rapidsai/cugraph/tree/branch-25.02/python/cugraph/cugraph/tests

pylibraft and raft-dask are also exceptions here:

• https://github.com/rapidsai/raft/tree/branch-25.02/python/pylibraft/pylibraft/test
• https://github.com/rapidsai/raft/tree/branch-25.02/python/raft-dask/raft_dask/test

Originally posted by @bdice in #584 (review)

[bdice]

I filed 3 PRs to RAFT, cuVS, and cuML to standardize our test directories to use the name tests.

• Rename test to tests. #590
• Rename test to tests. raft#2546
• Rename test to tests. cuml#6237