[Tests] Unable to run unit tests using PyPI release

The PyPI release is missing [`__init__.py`](https://github.com/G-Node/python-odml/blob/master/test/__init__.py) and [`util.py`](https://github.com/G-Node/python-odml/blob/master/test/util.py) in `test/`, making it impossible to run the tests.

Could these be added to the PyPI release, please?