[Draft] TEST: Run all tests through pytest

[david-cortes-intel]

Description

Currently, unit tests execute through a mixture of built-in unittest and pytest.

From the commit history, it looks like initial tests were added using python's built-in unittest framework, and then subsequent tests were introduced using pytest.

Compared to unittest, pytest has a much nicer output format, offers many options for how to execute tests and how to see their outputs, and has plugins that allow it do extra things which the built-in cannot when desired (e.g. JSON outputs, test coverage reports, etc.).

This PR switches the automated scripts to execute all of the tests using pytest. This doesn't change what tests were executed - just uses pytest as the runner.

One challenge here is how each of these interprets the path from where modules are imported - here I'm trying to control these with the PYTHONPATH variable when pytest is called within shell scripts, but don't know if this will work out for all the variants under which tests are executed.

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[david-cortes-intel]

@Alexsandruss Would you perhaps have any pointers about why the failing CI job here:
https://github.com/intel/scikit-learn-intelex/actions/runs/11218859163/job/31183555329?pr=2090

.. might be failing to import daal4py._daal4py in the new pytest call that I added, while it succeeds in the other calls?

[icfaust]

I've had to fight against pytest's package discovery recently and had to use --rootdir to get things to work properly (https://github.com/intel/scikit-learn-intelex/blob/main/.ci/scripts/run_sklearn_tests.py#L47). Maybe that could help you? I believe your daal4py._daal4py error is coming from the $PYTHONPATH which is using the repo daal4py folder to find daal4py instead of the installed site-packages daal4py, because it can't find the compiled _daal4py.so file there.