Add Transaction context in tests

Transaction context added by the wsgi dispatcher was not provided when
the NereidTestCase dispatcher was used.
Release version 2.6.0.3
Migrate Test model of nereid to 2.6 #1519

* Migrated test_nereid_module.pagination to 2.6

Cherry-picked from 7904853d7d80555dc6012bd3d4b64c18bce083bd
Release version 2.6.0.4
Nereid depends on Flask < 0.10
Release version 2.6.0.5
Use flash and get_flashed_messages from Flask

* The custom implementation within nereid are not required anymore
Release version 2.6.0.6