[QST/Discussion] Reorganize pytests

[ayushdg]

The pytests directory currently has a flat structure: test/all_tests.
It might make sense to move tests to subfolders based on the operations/modules they might be most relevant to? Eg: All IO related tests can move to io/tests like the pattern followed by dask or to tests/io like pandas.

[vuule]

I think for cuIO the preferable path would be tests/io/file_format so it's easier to selectively run groups of related tests from different files.
We can go ahead and make this change for cuIO tests in 0.19, unless there are objections to this structure.

[github-actions]

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[vyasr]

The plan for this will be discussed and decided upon in #11199