Allow loading cases for tests.py

[smarie]

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This is the issue fixed by your PR #344 from last year @last-partizan (time flies!)

I am lightly reluctant to support this officially, as the pytest strategy seems to explicitly mention test_ as the dicovery prefix by default :

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

It also seems that other users did not complain.

But since this behaviour was working in 3.8.5 and earlier, I will accept your PR.

My reasoning is as follows : "if pytest accepts to collect your tests.py file will the default discovery mechanism, then there should be a working AUTO behaviour". I will check though, if an error message can be issued for easier debugging...