test(python): More test restructure

[stinodego]

Relates to #6364

I did not change any tests, just shuffled some things around.

The aim of this PR is to better structure the py-polars/tests/unit folder. Currently, the folder contains more than 50 test modules, some containing only a single test, and others containing more than 2000 lines of code.

Setting up a sensible structure isn't that easy, as there are many overlapping concepts and data structures. This PR is not a final solution, just an improvement on the current situation.

Changes:

• Created some new folders and moved the appropriate tests there:
  • namespaces: I had already made this one - moved some more namespace methods there
  • datatypes: Test module for testing behaviour of specific data types in various operations.
  • operations: Test module for extensive testing of specific operations like join or explode.
  • utils: For things like show_versions and the like.