Fix AssertionError when AUTO cases are requested outside a "test_" module

[michele-riva]

Fixes #309

[michele-riva]

PR is good to go from my side. @smarie, feel free to add any suggestions

[smarie]

Thanks @michele-riva ! I made a few minor comments

[michele-riva]

Thanks for the suggestions, @smarie. I have just implemented them.

[smarie]

All good, thanks @michele-riva !

[mroeschke]

This broke a use case where we defined our "tests" in bench_*.py which was defined in https://docs.pytest.org/en/stable/reference/reference.html#confval-python_files and this worked in 3.8.1.

It would be nice if this used the pytest config to determine where to look for AUTO cases

[smarie]

I was not aware of this possibility @mroeschke ! And since I have been designing pytest-cases just for this (creating benchmarks) I really agree with fixing this. Let's move this to an explicit issue and discuss possible workarounds