avocado: Add mock library

Useful library to mock classes in unittests. Basically an
import from autotest with some small cleanups.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
unittests: Add datadir_unittest

Add one meager unittest that verifies part of the intended
behavior for the data_dir module.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Add data_dir documentation

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>