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/
Another option is to remove altogether this test. After all, it is a brain damage test checking that  the Python builtin `os.access` works. There is no point in this kind of useless tests, which are there only to make `coverage.py`. I leave the decision to the code reviewer.