Issues a warning when a function is not collected as a test case just because it uses @pytest.fixture (#12989)

[dongfangtianyu]

close #12989 .

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• Include new tests or update existing tests when applicable.
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A function is declared as a test case using the convention of the test prefix in its name ， or as a fixture using the pytest.fixture decorator.

When both are used simultaneously, it is not possible to have both a test case and a fixture.

So a warning is issued to remind the user: no test case, only a fixture.

Example

import pytest

@pytest.fixture
def test_function():
    pass

(venv) C:\Users\administrator\pytest\demo>pytest
============================= test session starts ==============================
platform win32 -- Python 3.12.0, pytest-8.4.0.dev262+gf160963c4.d20241211, pluggy-1.5.0
rootdir: C:\Users\administrator\pytest\demo
configfile: pytest.ini
plugins: hypothesis-6.111.0, xdist-3.6.1
collected 0 items                                                               

=============================== warnings summary =============================== 
test_aa.py:1
  C:\Users\administrator\pytest\demo\test_aa.py:1: PytestCollectionWarning: ca
nnot collect test function 'test_function',because it used the '@pytest.fixture' than becomes a fixture (from: test_aa.py)
    import warnings

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================== 1 warning in 0.26s ==============================

[dongfangtianyu]

Probably should add an ini config that allows old projects to ignore this warning?