feat: add warning when pytest.ini and pyproject.toml are present

[sgaist]

Add a warning when pytest.ini and pyproject.toml are present

The behaviour does not change however users will be notified that pytest.ini is selected over pyproject.toml and any configuration related to pytest it may contain.

Fixes #13330

[sgaist]

I am unsure what to do to satisfy codecov. Any hint would be appreciated.

[bluetech]

Thanks for working on this.

I haven't thought yet whether we really want this, but in any case a couple of comments:

We should not emit a warning if pyproject.toml exists but without a pytest section. That's very common and expected and not ambiguous (please also add a test for this case).

Also, it will be better not to have the "should warn" logic in terminal.py, it shouldn't know about config stuff itself. Ideally (I think) the core config file search logic should indicate whether need to warn.

[sgaist]

@bluetech I should have kept my original implementation which was following your suggestion minus the verification of the content of the pyproject.toml file.

I refactored the code following your suggestion.