testutils: fix global imports
- Addresses issue causing errors while running unit tests using nosetests or python interpreter from command line. (closes #1491)
- NOTE do not use global imports that require or might require application context in the future (e.g. SQLAlchemy models). Alternatively you can use invenio.importutils.lazy_import function that wraps string_import in LocalProxy object.
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>