[python][tests] small Python tests cleanup

[StrikerRUS]

• Rename temp model file to not overcomplicate .gitignore.
• Reduce code duplication in params definition.
• Use asserts from unittest module for consistency with the rest codebase.

[jameslamb]

I personally have a preference for assert with an arbitrary expression instead of the added indirection of the unittest builtins, but I wont block this PR

[StrikerRUS]

@jameslamb
This looks like a "good first issue" to rewrite all tests removing unittest dependency and referring test_dask.py and test_dual.py as examples. WDYT?

[jameslamb]

I agree! I can write it up

[StrikerRUS]

Thanks! Please do it. 🙂

[jameslamb]

Thanks! Please do it.

added #3732. Feel free to edit it if there's anything you'd like to change.

[StrikerRUS]

Thank you!

[github-actions]

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