patman: add pytest configuration file

With this change, a user can run the patman test suite using Pytest
the same as when using 'patman test':

    $ cd tools/patman && pytest
    [...]
    44 passed, 8 warnings in 8.87s

    $ ./patman test
    Ran 44 tests in 8.460s

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>