Move tests folder to top level for further refactoring

[bobleesj]

• Use conftest.py
• rename test file names from TestPdfFit.py to test_pdffit.py, etc.

[bobleesj]

Warning also removed: Based on forked CI result: https://github.com/bobleesj/diffpy.pdffit2/actions/runs/10983571527/job/30493096104

From:

=============================== warnings summary ===============================
../../../../../usr/share/miniconda/envs/test/lib/python3.12/site-packages/diffpy/structure/version.py:29
  /usr/share/miniconda/envs/test/lib/python3.12/site-packages/diffpy/structure/version.py:29: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    from pkg_resources import resource_filename

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================== 112 passed, 1 warning in 0.66s ========================

to

tests/test_exceptions.py ............................................... [ 41%]
.........................                                                [ 64%]
tests/test_pdffit.py ..............................                      [ 91%]
tests/test_phase_fractions.py ..                                         [ 92%]
tests/test_shape_factors.py ........                                     [100%]

============================= 112 passed in 0.60s ==============================

[sbillinge]

Super!

I think we need a news