Issue 12520: spurious output in test_warnings

I cannot reproduce this isuse, was it fixed?

$ ./python -m test test_warnings
[1/1] test_warnings
1 test OK.

> Also, I don't understand how test_filename_none is supposed to check for issue #12467 (it doesn't use a subprocess).

You don't need a subprocess to reproduce the issue, you just have to emit a warning with __file__=None. At shutdown, __file__ is set to None by Py_Finalize, whereas a warning is emitted because a file was not closed explicitly.