refactor: Refactor and reorganise tests

[EdAbati]

What type of PR is this? (check all applicable)

• [x] 💾 Refactor
• [ ] ✨ Feature
• [ ] 🐛 Bug Fix
• [ ] 🔧 Optimization
• [ ] 📝 Documentation
• [ ] ✅ Test
• [ ] 🐳 Other

Related issues

Checklist

• [x] Code follows style guide (ruff)
• [x] Tests added
• [ ] Documented the changes

If you have comments or can explain your changes, please do so below.

As mentioned in #321 , this is a proposal to clean up a bit the structure of the tests. I tried to keep the PR as small and manageble as possible, let me know if you think I should split it further. :)

The logic:

• follow the structure of the modules in the library: naming convention test_<modulename>. If a module has multiple test files it becomes a folder.
• anything else has <name of test>_test.py. With the exception of test_common and test_invalid.
• grouped all expression related tests in the same expr folder. Here there are subfolders for each expr context. e.g. cat, str. dt has every tests into test_dt.py.
• move to utils.py the code that was repeated: maybe_get_modin_df

Possible extra cleaning (that may make the diff bigger):

• rename expr -> expression, frame -> dataframe to match the module names
• create a folder for tpch unittest as hypothesis have (there is just 1 for now though)
• move frame/test_invalid into frame/test_common?
• anything else?

[MarcoGorelli]

thanks for doing this! much better indeed

happy to leave this to you two, feel free to merge when you're happy with it

[EdAbati]

"merge gifs" is one of the main reasons I contribute to this project 😄