avocado.core.data_dir: Contemplate read only dirs

The avocado tests location can actually be a read
only location, so take that into account when looking
for test dirs.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #59 from avocado-framework/fix-ro-bug

avocado.core.data_dir: Contemplate read only dirs