unittests.yml: python setup.py install is deprecated

[cclauss]

https://packaging.python.org/en/latest/discussions/setup-py-deprecated

Let's try pip install --editable . instead. That works perfectly.

---

Use pytest to run more tests and make test discovery automatic.

Rename the test files because pytest discovery will autofind tests/test_*.py files.

• https://docs.pytest.org/en/stable/explanation/goodpractices.html#conventions-for-python-test-discovery

% pytest --ignore=tests/test_islandora.py \
         --ignore=tests/test_islandora_check.py \
         --ignore=tests/test_islandora_hooks.py \
         --ignore=tests/test_islandora_paged_content.py

============================= test session starts ==============================
platform linux -- Python 3.11.8, pytest-8.1.1, pluggy-1.4.0
rootdir: /home/runner/work/islandora_workbench/islandora_workbench
collected 152 items

tests/test_csv_id_to_node_id_map.py .                                    [  0%]
tests/test_field_values.py ..                                            [  1%]
tests/test_fields.py ................................................... [ 35%]
................................                                         [ 56%]
tests/test_unit.py ..................................................... [ 91%]
......                                                                   [ 95%]
tests/test_unit_workbench_config.py .......                              [100%]
======================= 152 passed, 6 warnings in 1.20s ========================