testutils: fix for passing engine to app factory

• Addresses issues with inconsistently set database engine in application config since a80bc418.

• Adds new class InvenioTestCase that has to by used by _ALL_ Invenio tests in order to prepare proper application context for each test case. (addresses #1491)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>