added missing imports in db_tests/__init__.py (for nosetests)
added missing imports in db_tests/__init__.py (for nosetests)


Former-commit-id: 044a43a505df986377fe4eb117fc4efcacd1f0dd [formerly c8dccac82ce32a72e86de61e35a6b0b384c7ff62]
Former-commit-id: 96c7aa1e26ebaec8a93a1cedf2a32865ae082b28
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine