Fixed test_is_readable_all_files_lack_permissions when run as root

See https://bugs.launchpad.net/oq-engine/+bug/1392221.
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/831/
I will keep this, it doesn't test that just ```os.access``` works, but it also tests that a non-root user have permissions to read the ```openquake.cfg```.
If permission for the ```openquake.cfg``` are ```rwxr-x---``` and the owner is not the user who wants to run openquake, its execution will fail.